Quite a few threads here and about 100 at m.net... search for more info.
But since my name was mentioned earlier:
Foamectomy (2"-3" lower)
Bent seat rails (allows rear of seat to sit about 1/2" lower)
Parking brake handle mod (give 1" more side-room for right leg)
Steering column mod (stock seat, need to raise steering column... aftermarket seat need to lower)
No dead pedal
Package shelf removed (allows seat to lay back extra 2")
Door arm-restes removed (allows left leg an extra 3" of space)
And for the ultimate "let's make some room in a Miata"... the Uber-Seat.
If you really want to know how much room you have to play with, then simply remove the seat and sit down... I can barely reach the pedals and my eyes don't clear the instrument hood. There is plenty of room, you just need to maximize it. I actually thought about just getting some fairly heavy gauge sheetmetal and hammering out a dish for my ***, bolting it to the floor and padding it a little... then lean back against a pillow or something. The uber-seat is as close as I could come and still sorta feel safe strapped into it.
I'm 6'4" 250lbs... this is how I rolled when I had a Miata: