While others swear that you loose none, I am secretly conviced that you loose at least 1/2".
I can put my head straight on any coupe that I've sat in. I drive my T with my head somewhat sideways.
But these are just my impressions, I have no facts to back them up. One theory is that the coupes I've been in are earlier models and that the seat settles somewhat compared to my Y2K.
And note that Dali Racing sells a replacement cushion that may yield over 1" more headroom.