125-200cc Dreaded bog observation...

I'm running my bike as a 144 right now. When I got it, as a 125, it had the dreaded bog off idle. I did some reading and went with the '04 rm125 needle. With some jetting changes it all but eliminated it. The bog was just a narrow sliver off idle and then would be gone.
A month ago I put a FMF Turbine Core II muffler on it. I needed a spark arrestor for one of the tracks I ride at. It's at 3400' asl. With this set up there was 0 bog. I rode it at a track 800' asl and with a minor airscrew adjustment it ran great. 0 bog.
The other day I swapped out the FMF for the oem muffler, hoping to get a little more oomph at the 800' asl track. The bog was back! Is it reasonable to assume the oem muffler is more restrictive than the FMF with a spark arrestor? The oem muffler is a "closed circuit course only" muffler.
The FMF muffler is going back on the bike for all of my riding...

i would have thought id be the other way around? rich bog with oem on?

If the bog changes with exhaust, I would suspect that it is related to jetting.

I don't have any personal experience with the small bikes so I don't have any specific advice (sorry!), but I would suspect that the silencer flow differently and it's effecting the jetting.

The other possibility that I see is power valve, but that seems less likely for a silencer (more likely with a pipe).

update: I swapped back to the FMF TCII. No bog. I'm curious as to the why?...