Header Collectors

Yes, it very well could...

In high horsepower applications and in the upper rpm ranges...

BBK Longtube collectors are the ball/socket type. The collector necks down to 2.33", I measured it That means, there is only .17."

That is a tick over 4 MILImeters difference between it and a 2.5" aftermarket offroad H-pipe. In our applications, I would look elsewhere for added power. Just as well, the reason for the small inside diameter difference, is it helps the sealing between the ball, and the 'socket' of the H-pipe. Not all exhaust bolt-up exactly the same, do to different variables, so BBK left a little bit of play.

I know if I wanted to dremel mine 4 MILImeters out, I do not know if it would seal, because it doesn't match up straight on with the H-pipe.

Something like that small, I would look to if I was trying to get every single drop of 1/4 horsepower out of it

Some have did dyno swaps, and seen gains...

But, I have yet to see a back-2-back one

But either way, your thinking is correct in general