Well, I'll share my opinion. The canister part of the muffler is close, but not blocking enough of the fins to make a difference, IMO. There's something else going on, what, I have no idea. IF you are overheating, it usually is one of several things (pick any or all as they may apply)

1. Too big a prop= too heavy a load
2. Too lean a mixture = bad needle settings
3. Too little oil = no lubrication
4. Crappy oil = poor lubrication
5. Ignition timing too far advanced
6. Too little air flowing over cylinder heat
7. Too little air exiting cowling (see 6.)
8. Crappy engine with bad bearing, tight rod, bad ring, bad sleeve, etc.