I had a ST1000+ on my Capri 25, 3500lbs... spade rudder... on my Boat last year (used it for 2 seasons)...
I have a Simrad TP22 on my S2 7.9 4700lbs, transom mount rudder on my boat now, just this season.
Both are overkill for the size boat I had. 6000lbs I'd opt for the 2000+ from Raymarine, but I feel the TP22 form Simrad would be fine (doesn't break a sweat on my boat)... so basically it sounds like you have the right size... I'd rather the TP be slightly overkill than right at its limit anyway... 6000lbs should be no problem if it's a light tiller, on the ST2000+ and the TP22.
The S2 is a much heavier tiller than the Capri, its like a barn door hung off the transom, which is why I opted for bigger.
I feel if you aren't willing to do the basic calibration for tiller pilot, you are wasting your time. Also, I've personally had to fix the fluxgate compass on my ST1000+, its cool because it's easy to find Raymarine parts (on Ebay).. by contrast when I broke my TP22 (yep, less than 1 season, I broke my TP22, by me falling on it), I couldn't find basic parts on Ebay. That's bothersome.
OK, so on my ST1000+ I broke the fluxgate compass, by essentially setting the unit down to hard over and over again... I used to keep it on my cockpit seat, and would disconnect and let it swing aft against the combing. This 90 degree turn was usually when I had a handful, like when the boat just got a +10knt gust, and the boat was heeling like mad.. so you can bet the TP swung with a bang a few times... SO I couldnt' really blame Raymarine for it. The part that broke was a $1 plastic piece that creates the gimbal for the fluxgate compass... can you buy JUST that.. NO, so $95 for the fluxgate and gimbal (ugh). If I had a 3D scanner I'd have printed the damned part! Some more about the ST1000+, is that it'll do seatalk, and integrate will all raymarine stuff via seatalk (no question the 2000+ will as well). Calibration takes a couple minutes (I don't recall the procedure, which means it must not have been hard to do). I didn't have integrated electronics with raymarine for that boat, but I have no doubt it'd talk to raymarine stuff fine. That means you'll want a seatalk to NMEA 2000 converter if you have mixed brand electronics.
Ok, so my TP 22... the simrad TP10 is equivalent to the ST1000+ (in size, but it doesn't do simnet, or NMEA), but the TP22 is the same as the ST2000+ in size, and does both. I went UP in size because A) I knew I had a heavier boat, B) I knew I was going to have simrad instruments, and C) because I knew the barndoor rudder on the S2 would benefit from the stronger throw. So far the TP22 has never strained. The interface for the TP22, has 3 connections... POWER (for dedicated power), Simnet (which it does NOT derive power from), and NMEA 0183. The reason no NMEA 2000, is becuase SIMNET is basically already NMEA 2000 (they made a cable adapter, but you can splice a NMEA 2000 end right on the SIMNET cable, at least I can't see any reason why you can't, others can confirm this)... OK I like how simplistic simnet is, so I plugged it in to my IS20 Combi WIND, and was able to get it to talk to the TP22, without really having to configure a thing (trick is to have them all connected first before you first power on, otherwise you have to ADD the device in, not a huge procedure, but an extra step). The next thing was calibrate the TP22, its basically press starboard/port, and then tap TACK... do this while motoring in a large clockwise circle... you should be doing like 3-5 knots... it'll tell you if you are turning too slow, or fast by blinking green or red... Anyway once you make slightly MORE than a 360 degree turn... it'll usually be calibrated (positive beeps, otherwise one long beep, meaning try again). I only had it fail once and it was when I was waked by a motorboat. while doing it. After that it has done quite well. I've had to TP22s in 4 months (long story), so I've had to recalibrate it... which is why I had to recalibrate it.
OK... HOW do the 2 compare? I'd say pretty evenly honestly. I personally like the TP22 better slightly, but that may be more about the power it has (again I'm comparing 2 different sizes so that's not likely fair). The interfaces are drop dead simple on both (as it should be)... The steer to wind on the TP22 is pretty good (can't say for the ST1000+ as I didn't have those functions in instruments), but almost useless on our mountain lake, as wind shifts send it reeling pretty quickly. NOt because it doesn't do it, it does, but it sets off alarms on my IS20... which I've not turned off yet. But it works, and tracks like mad even with 90 degree shifts, but if you get a 90 degree shift it's really a hole, and you'd rather it blindly steer through it, cause it's coming back anyway.
I've felt comfortable slapping the TP22 on even while I'm heeling at 30 degrees right on the edge of 30 degrees off wind, so that I can go below and use the head. I've only once had it (while steering a course not steering to wind) have me round up... but the reason was winds shifted 15 degrees as a header, throwing the bow over.
Obviously I am not much help when it comes to sea state, as I'm not in open water. I've been waked more than my share of times, both under sail with TP in control, or more commonly with me buttoning up the boat while the boat was motoring back to the marina. It handles it quite well, and reacts quite quickly. I wouldn't want to have a boat next to me (less than 50 feet) while it happens, as the boat might correct back and forth some... yes I've had the TP on while motoring less than 50 feet next to another boat (I was taking pictures - again another long story).
I'm going to dig and see if I can find a few videos with the (each) of the TPs under control. I don't think I have many with decent levels of wind, because I usually like driving in those conditions, but I'll see. I am sure I have several with 8-12knot winds. I doubt I have any with white caps. I'll update THIS post if I can get them up on u-tube.
If you have specific questions I'll try to answer what I can. Again, I can't vouch for how they'd do in following seas (I'm on a puddle)... but I CAN tell you that they correct the most downwind. It takes them a while to adjust to the state while downwind. The TP22 seems to correct instantly downwind when you turn it on.. adjusting 5 degrees or so immediately the wrong way. This is unnerving under spinnaker, because if it's blowing hard enough, its a self-perpetuating rounding if you aren't careful. Best to be ready to trim spinnaker when you first turn it on... but it recovers quickly and after that its fine.
This is the TP22 in light air (again sorry no heavier air)...
I'll endeavor to get a short video with my new phone (better HD video) hopefully this weekend upwind.
This is the ST1000+ in slightly heavier air than above, on a close reach (better idea how it handles).
Others here could tell you more, but as I understand it, you can add a "rate compass" and highly improve the speed at which the TP reacts.