For me (OP), I have to be honest: I no longer track the car, and I find the 986S to be absolutely delightul for 7/10s or 8/10s street driving in the mountains. When I spent a good bit of time driving the BS last summer I found it to be slightly evolutionary, but not at all a "revolution" in terms of driving dynamics. I'm sure it must be at 9/10s or 10/10s, in the proper setting where that difference can be exploited. But for my purposes, I didn't find enough difference to prompt me to change. Plus the bikini top was a deal breaker.
So the only real reasons I would upgrade to a 987S would be:
-- appx 2" more legroom. When I set the seat up where I like it in my car, I have knees brushing the knee bolsters. In 987, the same setting has knees 2" from dash. This is a very good thing for me on longer trips.
-- 9A1 with no IMS. (Of course, who knows what HPFP or other issues will develop because of new DI tech, but at least right now they seem to be pretty bulletproof.)
-- Could change color inside and out. I love mine, but it's been almost 8 years.
-- Probably some warranty left.
-- Better stereo with iPod in.
Reasons to go to 981S:
-- On screen so far it has looked really gorgeous to my eye. I think I love the new design.
-- 9A1
-- Color/interior change
-- More leg room, possible more trunk room (don't know if that's true.)
-- Probably about the same weight as 986S.
-- Long warranty period before I start thinking about expenses again.
-- Better stereo that has iPod in.