In retrospect I'd say you are right.

When I made the flange I drilled 2, 1/2" holes in a piece of 3/8 bar and cut between the 2 holes.
That was about 3/8 of an inch.
The actual I.D. of the tubing is .630. I cut the ends at a 45 degree angle so they would scavage better
and put them in about 3/4" from the top of the muffler, faceing the muffler walls.

At the time I believed that the rings had not seated yet.
But listening to the video a few times I can see where the exhaust could be choking the engine down.

Anyone know where there is a flow chart for tubing?