I'm guessing you've paddled at least some of the boats in topic heading - SEI, Zeppelin and CLK - last one I had to look up
www.surfski.info/forum/19-boats/9819-genius-clk.html
Like you Gnome I'm around 85kgs, and though 180 cm am stumpy in the legs. I've paddled an Evo ll and own an SEI and SES so will give you a rough comparison choppy river only. No Garmins, no racing, only seat of pants stuff. Most of my paddling is in my SES.
See Wesley's chart
www.surfskiracing.org/surfski-reviews-updated-march-2012/. Latest update is Dec 2103 despite the URL. For me I felt the SEI to be more stable again than the EVO ll (not the Evo) compared to where Wesley has it. His chart has the Evo ll marginally less stable than the SEI and the Evos around about same stability as SEI. On that I can't comment. I find the SEI very very stable however some mates struggle in it, others - wife of a mate the other day on very first paddle didn't fall out. Go figure. I would have no hesitation strongly recommending it as an all rounder.
As far as speed goes I liked the speed and feel of the Evo ll (the Ion even more). Frankly compared to my SES at 11kg the SEI at 11.5kg feels a lot heavier in the water. I suspect this may be the double venturi and extra width compared to my SES which is effortless. Can's say the SEI is slow though as I've paddled with a couple of guys in SEI's recently and I was in the SES - my arse was fairly hanging out just holding on. So speed thing there as much about the paddler as anything else. Sits right up there in Wesley's chart anyway.
Cannot make any comparison with the CLK.
As far as the rough goes, went out couple of k's in the SEI on weekend blowing a choppy 25knots with lot of refracted stuff off a Marina (interesting it was all over the place for about a kilometre off the wall) and I found the SEI a joke in terms of stability. Yes, that's coming from an SES nonetheless if you done a bit of paddling the SEI is easy street. Positives - it sucks baby - from about 7 or 8 knots it's venting out. It's also surprisingly narrow internally despite its stats (marginally wider than my SES) with something akin to cutaway sides but it's still too wide at the catch compared to other boats. Internal space reduced from enormous volume of SE / SEL and to an extent the SES, however all Stellars need to be re-visited here, very poor comparatively.
Build is very strong, fittings excellent, rudder adjustment exceptional, comfort high though the back of the bucket more laid back than I'd care for. Think Ion is best however no complaints with my SES.
Hope that helps some.