'Every 180 degrees is for 4 into 1, which gives more power at redline . 4 into 2 is every 360 degrees, which gives more power in the more useful mid range.'
No. Don't know where you got that. Actually, likely I do but it's wrong. I can make upper end or mid range power with either one of those setups. Each one tends to lean one way but they absolutely do not have to. I've built 360 degree systems that made power at over 10,000 rpm and 4-1 180 that makes power at mid range to pull your face off and dropped in power at higher rpm as intended. I can tell you why the 2 timings go where they do but it's not a high or mid power thing, rather the overall useage of the system to best effect. You can do the exact same with a single cylinder engine.
X pipe is NOT overkill, it's a crutch for a system that is not optimum. They actually weaken pulses.
And what about the standard Japanese 4-2 that comes on most bikes with fire at 180, fire another 180 and then dead for 180 and dead for the next 180 too? Using 1-2 and 3-4 paired up. It's not either one of those.
'I don't see a 20% drop in power.'
No wonder, your exhaust ideas are sadly mistaken. I've run the smallest engines you can think of (Yamaha Twin-Jet 2X50 cc., 9.5 hp. output, modified to 18 hp. do the % calculation there!) to the biggest that go in a car (700+ cubic inch, 1500+ hp. in a 200 mph pro stock race car) and messed with exhaust on all of them, I'm considered pretty good on the subject by those that know me. I build expansion chambers for 2 strokes as well. I can make a 4 stroke setup on a stock DOHC that picks up 10% just by removing the fiberglass packing around the muffler core. BTDT. I raced a Kawasaki 400-3 modded from 40 to 66 hp., the exhaust was the reason I got it so high. Stock engine hp. at 7000 rpm, mine shifted at 10,000.