I want to autocross it, but my head is touching the sunroof frame as it is, so a helmet doesn't seem realistic. Any clever tricks that I can employ, or do I have to get a hardtop (seems like they're hard to find)

 

A Sparco or similar racing seat will make you sit lower.

 

lol.... recline?

 

I had the same problem. I'm 5'9", and with helmet on, my head was canted to one side all the time, and the roof had to be open (a drag in the rain!).
With your seat removed, there are two things you can do: 1st, stretch out the wavy springs under the seat bottom, to make them a little slack. 2nd, undo the covering on the seat bottom and peel it back until you can access the foam of the seat cushion. Cut out as much fom as you need / dare. I took out a bowl shaped section about 3/4" thick, still leaving about a 1/2 " plus what's sewn to the cover. Together I gained about 1- 1/4" head room - I can sit up straight now with a helmet on ( and I like the seatback up, not reclining.......).
Undoing the seat cover is fairly simple, I did it as part of a reupholstery job, but basically you cut the 'hog rings' that attach the covering to the frame. Mark where you remove them from, when you put it back together just use zip ties.

Good luck,

 

or you can take out the head liner and sun roof. I think il be doing that. Ill make a new head liner out of CF or somthing, and a non openable sunroof insert. if anyone is intrested in this, i guess i could make more.
I have the same problem when i race too.