Opium with 3 crew is lighter than Stream with 10 crew. We talk a lot of weight versus early planing. When going downwind I maintain you have no or very little use of 10 crew so Stream heavier and less optimised for planing. And since my comments, as stated, are about cruising I am not saying a boat will not be faster on a beat with both a lot of ballast AND weight on the rail. But most of us cruisers, even if performance oriented, are satisfied with good performance beating and are looking for excellence on other angles.
Opium does not have the sail areas you mentioned, my areas are upwind 82 sqm and with gennaker 172 sqm.
A boat with narrow hull and calculating with crew weight on the rail for RM and with singel rudder will be difficult to sail with small crew cruising. As you said yourself the boat need to be sailed at 80% and especially in an archipelago where you need 110% behind the islands and 60% in the gusts and open water you will have a lot of broaching.
And where all this weight discussion started was my comment that we 3 in our crew, with a weight of 200 kg can bring a lot of cruising gear compared with a similar boat with a bigger family or racing crew consisting of many heavy persons, a fact I only wanted to point out was missing in many comparisons.
Anders - I'm not sure your factoring in what is required to sail the boat at 100% optimal performance under given conditions. Let me explain...
In marginal planing conditions, the weight of the extra crew is definitely an obstacle to breaking out of displacement mode and achieving planing speeds. In such conditions, fewer crew are required to handle the boat in order to achieve optimal speed.
However, as the breeze increases, so does the boathandling effort. I can tell you that everyone is busy on a 42-foot boat going downwind with mast head spinnaker in 20+ knots, particularly as you achieve planing speeds. At this point, apparent wind goes forward and the boat wants to round up, so the crew find themselves again in hiking position towards the back of the boat, to counter that effect.
Of course, we know that boats like the IMOCA 60 can be sailed in such conditions with one or two crew. But this is because they have been optimized precisely to compensate for the missing 10-12 bodies, via canting keels, water ballast, coffee grinder winches, etc.
Boats like the Stream 40 don't have water ballast or canting keels, so they have to make do with crew weight, which is effectively movable ballast. The trouble with cruising boats in planing conditions is that the equivalent weight is generally not located in the proper place to optimize boat speed. If you could take your inflatable tender, flat screen TV, water maker, and other items of weight, and quickly reposition them to the aft quarter of the boat, you'd achieve the same effect as the stripped out performance cruisers do with 8-10 handsome crew in matching Gaastra kit.
I recognized a short time ago that Mills should probably get a bit more credit than Ker for his IRC designs precisely because Mills makes an effort to provide some semblance of a cruising-friendly interior for his boats; Ker makes almost no such effort, but chooses to exploit other aspects of the IRC rule to achieve racing success. Still remember setting foot on Mills' IRC 37 "No Naked Flames" at 2008 Key West Race Week and being shocked by how comfortable it was down below, in contrast to its very racy (and sexy) exterior. The Swan 42s were there that year and looked sluggish by comparison.
Anyway, Paulo is correct, I think, that boats like the Redline 41 and the Ker designs can be sailed shorthanded if you make the necessary accommodations - i.e., you're going to reef early and switch headsails early and use smaller kites, etc. Ultimately, though, you won't be able to sail those boats to their optimal performance level even in the lightest breeze, since you'll be missing those 8-10 handsome crew down on the low side, inducing leeward heel, or tucked down below in the forepeak, reducing stern drag.