I'm 6'4" and I've got a '99. I've put in a racing seat that lowered me by about an inch and a half. You can read about that experience here:
The truth about GT3 seats for tall drivers
For track days, I just got new floor mount adapters from Brey Krause and another set of side mounts. I'm going to custom drill them so I can mount my Recaro SPG XL directly to the floor. That will get me about another 3/4". In fact, I learned that by getting rid of the sliders, I can actually mount the seat reclined a bit to get my head even lower. I lose a bit (under 2") of leg room, but gain about another inch (purely a guess) of headroom.
The reason I'm going through all this is because with a helmet on, with the racing seat, and a BK roll bar extension (which I don't have yet), I still don't pass the broomstick test. Forget having an inch to spare, I don't pass it by a hair.
Your concerns are valid about roll overs though. Even though the car does slide pretty stably, and it corners damn flat, if you go off the road in a slide, it will roll. I try hard not to push it that hard though; certainly not on the street.