Looking where I can shave, I took the back cover off the seat. It was a royal pain and I still ripped the foam. Honestly, the only thing that can be gained for support is to remove the lumbar without modifying the rear support springs/cables. All the gains are in the lower cushion.
Tools
-Phillips
-10mm socket with 6" extension
-10mm wrench
-Fense Cutters [Hog Tie Remover]
Directions
-Remove the seat from the car.
-Cut Seatbelt switch zip tie from frame. Remove attached frame.
-Remove bottom plastic side cover.
-Cut hog ties holding leather to frame. You can try to salvage these if you really want but they are a strain on patients and much easier to replace with zip ties. Near the back corner mounts there will be a two hog ties in the same area to be aware of. Upon removing the back flap, there are another set of hog ties on the inside seam.
-MAKE SURE TO CONDITION THE LEATHER BEFORE DOING THIS. Pull the back flap toward the front to give some slack to work the front flap off the foam. Will probably have to tilt the back forward to get the room. There will be a wool layer on the backside of the leather. Peel it with the leather into itself to make a bowl shape.
-More hog ties hold the inseam of the leather to the foam, 3 along the inseam of the wings each and 3 [or more with heated seats] along the first cross inseam from the front [look at pictures for reference]. Leather should just peel off now.
-Foam is glues to the frame so, run your finger along the inside lip all around while working the foam off the frame. Tilt the back forward to get the back foam lip off the frame and move the foam forward and off. Heated seat wire, for the back, will run through the foam. Use a small flat-head screwdriver to remove the connector to remove it.
If I still had a sewing machine I would consider cutting out all the center foam and make a custom removable insert.
Bare Bottom Frame
There is 3" of foam to work with from the "1st left-to-right 1/2" cut from the front" to an inch past the rear leather attach points. The foam compresses only so much since air can't escape. By cutting and not removing foam more compression is allowed while giving support. After testing and refitting, this is what felt comfortable for me.
Using a straight edge/ruler, pen and box cutter blade is all I used.
This is just what I did and how it worked out, use it to make your own if you want.
Acutal Picture
The 15Lbm bowling ball used for comparison [2" drop]. Sorry for the bad lighting.
Head still strained by the headliner with a helmet but only if I stretch my neck, otherwise I use the sunroof recess. It just brushes with my head in a comfortable position.
I haven't secured the leather back to the seat yet, looking to fix frayed stitches before doing so. The seat gives more support for cornering, but that's like comparing the balance of an egg on a knife to a shallow spoon. Probably could do some angle cuts into the foam so the leather sits better, but I am happy with the result.
