there's no harm (except to your eardrums) running no muffler to test that .. I and a few other people around here have done it.
What should happen is top speed increases since exhaust exit is much improved at top RPM. However, you lose lots of fresh mixture out the exhaust and at lower RPMs, ow-speed power will suffer badly.
My top speed went from 30 to about 37 MPH without the muffler.
yeah.. do a compression test. You could have any sorta problem on the top end .. damaged rings or rings stuck in their grooves.. a holed piston .. a warped head .. who knows.
Dont be shy about pulling the head for the sake of a freakin head gasket. New ones are easy to make from sheet aluminum or copper.
You've checked so many things that it might be time to get serious and check crankcase compression too .. might have a leaking mainseal.
A leak there will show oil residues on the outside of the engine near the main bearings. Sprinkle some baby powder around the bearings. If it turns dark oil being blown out the seal is making it dark.
The crankcase should hold about 7PSI for at least a few minutes.
Seal all openings and use a small hand pump. Seals might blow (be ruined)) with more than about 10 PSI of pressure.
A lot can happen in 1,000 miles or even in 10 miles .. Like running without lubrication can do amazing things pretty quickly.
Pull that head and look-see whats up in there.