Ben, lovely suggestion. Reminds me of when I did something similar with vertical tail area and CG positioning on a 2 meter aileron glider. I learned SO much about the relationship of spiral stibility, vertical tail volume and CG placement in one afternoon that it was like an epiphany.

Qfrc, you'll probably be doing some surgery to replace or otherwise build up the stab and elevator anyway so adding scab pieces onto the exising unit for some research time won't be a big deal. You and we may learn a LOT with some testing.

Add lots to bring it up to about 25% and then cut it back in 5 % steps to the original size and keep some notes about where the biggest change occurs. That would be the critical size that you're looking for.