I won't pretend to know how to adjust the exact numbers but exhaust is a bit different than intake. It's all about pulse timing and gas velocity in a pressurized system whereas the intake is a negative pressure system.
I think you could adjust things the way you suggest but not to the extent you suggest. The disks adjust back pressure only. The pipe length is tuned to function properly at certain rpm ranges and frequency of exhaust pulses.
In general I think the smaller OD you use, the shorter pipe you can use but I'm sure there are limits to this. The sizes I shoot for 24-25" head, 15" mid, 24" meg (1.25" to 2.75") are based on 1.125" head pipes... and I use 1.25" so I'm off a bit there, that's why I've tried to keep things at the upper end of the length if possible.
Off hand the shorter system you describe doesn't sound like it would hurt power that you or I would notice. Low end you lose with a shorter meg you'd probably make up with the smaller OD head pipes. Not a bad idea. The disks adjust back pressure, which affects power band, but length does the same thing for totally different reasons.
You are thinking in the ball park now, I'm betting the system you describe and the system I use wouldn't show a 1/4 hp difference on the dyno... might move things around a bit, but no real gains or loses. And nothing personal...but anything close has got to be better than some of the things I've seen bolted to your bike in the past
The measurements I've given you in the past are just that..ball park. Not bad enough to hurt you, but probably not optimal either. I've dyno'ed 4 different systems now, and none of them made a substantial difference in peak hp, decent gains were made in mid range with the system I use now though. I don't have the dyno evidence to prove it but on track experience tells me the best change I've ever made to the exhaust was to go from 12 disks to 7....and I did it just because I was pissed about failing the sound check.
The meg is what I would screw with the least, it's a functioning, balanced kind of thing. Besides...the one I'm making you is about 23.5" and 2.75" OD at the big end. I KNOW these work well on Mary's bike and mine. Shorten the mid pipe if you have to, or knock some off the small end of the meg and slip it over the exit of the mid pipe. I know you've already made that mid pipe...but I can't tell you how many times I've cut mine apart and redone them. I'll send more tube. And yes my meg points more to the sky every time I build a system. Hell, look at Roper's...it's almost vertical. I think I have 20" megs in stock, already made if you'd rather I send something a bit shorter.
They are 1.25" x 2.75" x 20".
JohnnyB