I believe the traditional thinking is that a 2S .61 is roughly equivalent to a 4S .91, (based on what you always see on the side of the kit box), although for aerobatics I agree that's not the case in reality..

In addition to the obvious choice of 4S engines, (no air chambers, supercharging, or oversized carbs), for noise reduction--everybody quickly took full advantage of the "verical advantage". Don't let the "SENIOR" part of SPA fool you--guys out there love to compete, and will take every advantage within the rules to get a performance edge, including 30% heli fuel. The main difference between the old AMA patterns, and SPA is the deliberate decision to limit and specify engine type, and landing gear configuration. This creates a natural "technology leveling effect", and performance ceiling, (which I believe to be more than adequate). The emphasis is then shifted to flying skill rather than "pressing the envelope" technology-wise.

4 Strokes are pretty much entrenched in the psyche of the contestants. I had one of only three 2S at the meet I attended. There was a certain 2-Stroke comaraderie out there, and we fancied ourselves as the purists. The 2-Stroke is what I am most familiar with, and the sound is the one I associate most with the original SPA era--but as you say, vertical performance isn't competitive.

Personally I would love making muffled 90s legal to "even-up" 2S and 4S performance, but that might open up a real "can of worms" I'm afraid. If a muffled 90's performance exceeded a .91 4S, there would be the inevitable push to 4S 100s or 120s.

It sounds like a good idea, but it would alter the deliberate decision made "back when" that makes SPA so unique.