Not experienced this with mine but from previous threads here, this looks very much like a clogged catalytic. Then I'd proceed to a disconnect and test a bit of extra noise

....you know, the exhaust CANNOT accumulate...ALL the exhaust will eventually go out, otherwise the engine would not run at all....but getting it out with a restriction will require more (back) pressure (thus restricting the intake) , which is precisely what you said ..
There might be a second explanation..but probably not as impairing as what you describe. Knock sensor failure would set a default mode that retards spark...hence less power, but the clogged catalytic is my best guess.

Here's even another hint from "brettclancy" on another forum:
"I removed the #1 converter only to find it mostly empty. Seems the guts of the cat have crumbled and collected in the exhaust pipe between the #1 and #2 cats. "