Lowering my stock t2 seat
 

Here's what I have learned.

First off, I'm 6'3". Some people are leg tall, others torso tall. I seem to be both.

Aftermarket seats are not lower with pre-fabbed brackets. In fact, most cases they sit higher unless they are side mount race buckets because they have firm flat bottoms while the stock seats curve downward between the rails. I have some Corbeau seats that were supposed to be low for my 1st gen. I used the supplied brackets from Corbeau and they were at least 1" taller. I ground off the rear tabs from the supplied brackets and cut out the rear mounting perches in the car, then mounted the rear part of the bracket directly to the floor with some over sized washers and bolts typically used for seat belts. I had to space the bracket up with some nuts on the bolts so the sliders would still go all the way back. End result, maybe 1" lower than stock, just enough for me to clear with my helmet on...but I really wish I didn't have a moonroof model.

If you like your seats and they are supportive enough, cut out the rear perches on the floor, bend the bracket tabs backward, and mount them to the floor. That should give you about 2" total extra headroom. If you don't think that is enough, shop for a side-mount seat you think you would be comfortable in and mount it directly to the floor.