As a 6'3" A4 owner I never had any headroom issues with the sport seats and a sunroof even when I stuffed my fat head into a Simpson helmet and took the car to the track. "Plenty of headroom", I always beamed. Then came my fast, stealth, power seat appointed S4 with seats that sat a lot higher than the A4 even when lowered all the way down. Uh-oh. Headroom was still not an issue for daily driving but with a helmet I constantly bumped the roof. I discovered that if I sat slightly crooked (akin to someone cruising the boulevard) the opening of the sunroof, with the shade closed, offered just enough clearance. Problem was when I took a hard right my helmet would bump the edge of the opening. Annoying and distracting, yes.
I took the seat out and looked at the seat mechanics to see if I could find any way to get the seat down further, no dice. When the seat is all the way down the seat lift mechanism is fully retracted and the screw trolley is butt up against the transmission for the worm gear housing. Bummer.
Not ready to give in I went to a trusted auto upholstery shop to see if they could trim some of the cushion foam without making the seat look funky. "No problem", he said, "$80 to do it right." I'm a mechanical minded guy but working with interior trim is my least favorite car thing to do. I usually have poorer than expected results. Naaa, for $80 it wasn't worth the risk of ruining my (I'm guessing) very expensive seat. I gave the green light and requested they remove 1" of foam. The result is excellent. The seat looks normal but when I sit down the world looks different. For the first time I can see the 100 mph mark on the speedo without scrunching down to look (uuhh, not that I'd ever be going that fast...). My helmet no longer bumps the roof and I can get my fingers between the helmet and roof if I put the seat all the way down.
There's only one catch to the modification. If you're one with uhhh... girth (34" waist here) be careful how much foam is removed. The seat bolsters are angled so the lower you sit in the seat the more they contact your thighs--too low and the bolsters will become pressure points on your thighs. I'm more firmly secured in the seat but not enough to be troublesome.
After some seat time (no pun intended) I found that the side bolsters became pressure point on my thighs at about the middle of the seat. I'm not lean but I'm certainly not someone with a sizeable buttocks or cottage cheese thighs but there was just a little too much side pressure to be comfortable on long trips. Despite my earlier fears of modifying the seat by on my own I decided to take on the task of fixing the side bolsters to my liking since I didn't feel like parting with another $80. After I saw how the upholstery shop took the seat apart I felt confident I wouldn't have a problem. So, I took the seat apart. My solution was to take a small amount of foam out of the out of the side bolster area and to adjust the position of the side bolster rails which are welded onto the seat frame.
I've decided not to put up info about taking the seat apart because it's equipped with an air bag. The service manual has warning after warning after warning after warning in big bold red type about the proper procedures for working on seats with airbags. Have a pro do it for you or consult a service manual before you start taking the seat apart.