There will be zero gain in power levels by cutting the Bisson muffler tips any farther up that what it takes to remove the crimped tips. All you'll get is shorter stacks.

7,200 +/- a little is about right for the Mejzlik 22-8, a little lower for the wood Menz-s is also about right. As the engine breaks in there will be a continual rise in the rpm until the break in process is complete. That rise will not be a lot. Better to tune the engine for reliability and best power that to try tweaking it to get the last possible rpm out of it. That's how engines get fried. That and a lot of ground running at high rpm levels.

Don't forget that no two people will ever have the same rpm numbers with similar engines and prop. Density altitude plays a large part is the maximum performance output. If it's hot and humid you will not get as much power as when it's cool and dry.