The OE bulkhead is laminated.

Which already helps.

There is black

fire retarding material sandwiched between each side of the firewall ie in between both sides.

On my RS200 replica, I've added 1" thick black fireproof insulation to the sides of the rear bulkhead and all across the inside of the cabin side, then added a further carpeted shield over ply.

Sounds a lot of work and it is, but is it worth the effort?

The prevention is lost if the windows are open and noise also comes through the rear screen.

A cone filter makes a hell of a high pitch screaming noise on boost or a high revs also.

If you want a quiet MR2 wrap the std airbox, and run a quiet silencer!

Good luck
Mike N