Update on JB progress:
Headphones working on some ROM variants, so that's a plus - only a matter of re-compiling to incorporate fix into the kang I'm following.
Microphone doesn't seem to be working for any JB alpha as yet, but there are half a dozen or more devs working on that issue. Once solved, that will mean the in-call audio will be active and the phone will make and receive voice calls. Also, Google Now doesn't work for the same mic-related reasons - however, it functions if you type your query.
Various graphics bugs are being squashed one at a time, and FCs are being sorted as they show up.
Still a ways to go, but getting closer. Don't recommend trying out the alphas unless your Sensation is a spare phone.
I didn't get the "buttery smooth" improvement on mine when I tried JB, but it's still alpha, and TBH, my Sensation is really smooth anyway.
More to come when there's further progress.