You may have a problem if more data arrives at the high-speed side of the converter than can be delivered at 4800 - like overfilling the big end of the funnel, some of it doesn't go out the narrow end. Unless the converter is smart enough to recognize and drop full NMEA sentences, it will likely just drop data whenever it overflows which may chop up the sentences you want.
I use a Raymarine E85001 to feed NMEA to a serial terminal server on my internet feed (to check on wind speed/dir/temp when the boat is in winter storage, as it is now), and occasionaly plug in a serial-bluetooth adapter to drive laptops. I do have an Actisense mux which combines radio DSC (no AIS yet, but soon) with the autopilot "fast heading" output to drive my E80/E120. The mux and E85001 are not connected however; they are basically separate NMEA networks.
Originally I tried the NMEA output of an ST60 Graphic to drive my internet feed and laptop, but for some reason none of them (I have three) would provide all the sentences the manual said they would. The E85001 does a better job. As they seem a bit fragile, I did buy a spare (both on eBay).
In the "gadget" category, here at home I currently have the spare E85001, a spare ST60 Wind (rendered spare when one failed and I replaced it while having the first one repaired) and an ST60Multi (which was replaced with a Graphic aboard) plugged into a serial/ethernet box configured to autoconnect to the one on the boat. So by turning on power, I have a Wind and Multi on my desk echoing what's being displayed aboard, within a few seconds of real time. Somehow make the boat and spring launch date seem not quite so far away.
1998 Jeanneau SO45.2