Today I drilled out two 11mm holes in the muffler cap. I also bought some very short shank bolts with lock washers and nuts for those holes.
First test was the two holes open. Loud. A little less loud than with the cap completely off. Second test, one hole closed. Ah. That's about it! A little louder than no hole at all. But OK. Good response twisting the throttle. Good sound.
So now I have 4 power choices. lol
2 holes closed. 1 hole open. 2 holes open. Cap removed.
For me the one hole has basically solved my problem. I still want the SBP pipe...but not as urgent as it was a week ago.
I don't recommend anyone drilling holes in your muffler system unless you really know your motor and what you're doing. Even as much as I know about my motor I was still taking a chance by drilling holes. Turns out I'm good!
My regular ride up to Yang Ming Shan required a few sections where I had to pedal. That's just fine. That's where I want it to be.