I'm 6'4", 35/36" inseam, 210 pounds. I fit in the 986 Boxster pretty well. I did do the GT3 console delete kit to give me more leg room. It was fine while driving day-to-day, but on the track my hand was hitting my leg. The console delete fixed that. Mine is a '99 so it doesn't have the headliner in the conv. top. I have enough room. With the hardtop on, which does have a headliner, my hair brushes the headliner which is very annoying. I've put in a racing seat for track use to give me extra clearance with my helmet on. That's lowered my seat about an inch and a half. I've lost about 2.5" of leg room, but I'm still comfortable in the car.
BTW, the other reason I put the racing seat in the car is because the stock seat killed my back. It had nothing to do with room, but everything to do with lack of lumbar support.
Although your head will be above the top of the stock seat in the car, it will not hit the rollbar in an accident. It will hit the rollbar "padding", which is actually hard plastic. I'm guessing that it is designed to absorb energy so that your head will not hit the rollbar hard. It will still hurt though. My racing seat has a back that is high enough that my head will hit the seat rather than the rollbar.
When I got the car, my wife was "okay" with me getting it. After I let her drive it a couple of times with the top down, she now loves the car.
If you decide to go with an aftermarket seat to get more head room, send me a PM and I can tell you exactly what you need to do. You can also read about my experience in the thread titled "The truth about GT3 seats for tall drivers".
Oh yeah.
AFAIK, the 987s all have the same amount of head room, regardless if they are a Boxster or a Cayman. That is slightly more than the 986. I've never sat in one where I could play with the seat position though, so I can't tell you how much difference it makes.