I found that the GT3 seats gave me less headroom than the stock seats only because they are the same height (with sliders), but you sit so much more upright. If you can go without sliders, then any racing seat should give you a bit more head room. If you want sliders, the Recaro seats will give you about an inch and a half more headroom. However, if you go with the SPG XL (which is the only option for a lot of tall guys), you will give up about 2.5 inches of legroom. The seat is so tall that it hits the rollbar. The nice thing about that is you head will never hit the rollbar, it will hit the seat back.
I detailed my findings when searching for a lower seat solution in the following thread:
The truth about GT3 seats for tall drivers.
One thing I did not mention in that thread is that the headliner in the hardtop is a bit lower than the early soft top. I plan to modify mine to get the same amount of room.