I have never had those problems.
Would you know if you had though? Typically it's a very slow progression, you aren't going to lose 20HP over night, it's more like over the course of years and you'll have zero memory of what it felt like before that point so you'll have no idea that you lost it. I've torn down heavily varnished engines and observed what I described, that's why I stated what I did and why I noted that the definition of "running fine" can be pretty broad.
Personal anecdote (
) When I got my '88 F-250 with the 300 I6 it ran fine. By running fine, it was pretty slow but idled smooth and drove around well enough, just didn't have very much get up and go. My grandfather had always complained about it but had assumed nothing was wrong with it; it was just the way it was. My aunt, who I bought it from after he passed was of the same mind, that it was just a truck and that's the way it was.
I had owned it for a couple of months and one cool fall morning I noted that the steam coming from the exhaust was, well, it didn't look right. There wasn't enough of it. My Mustang threw a good plume on those fall days and there was barely a wisp coming out of the tailpipe of that truck. I had my wife rev it, the plume did not change. Lightbulb! There was an impediment in the exhaust! Being a multi-cat system, it had a large 2nd cat that both manifolds terminated into. I hacked off the pipe behind the cat, took a prybar to the inside of it and fire it up. It blew cat material way behind the truck and immediately you could tell it was much freer. Installed a section of pipe to hook the rest of the exhaust system back up and it was just like the Mustang with a big old cloud of steam coming out of the pipe that grew in size when rev'd. It was like a whole new truck! Picked up a massive amount of power (relatively speaking).
A good friend of mine, his Mustang (stock) never trapped anywhere near as high as mine did. Mine was clean inside with factory-spec oil pressure, despite high mileage, his was lucky to show 15psi and was UGLY, had lots of varnish. I was cutting close to 100Mph stock, he was down around 88-90. We eventually tore his engine apart and observed what I described to you. We knew it had some blow-by, but the oil control rings were stuck on almost all the pistons, the rings lands had heavy varnish and the rings were sticking. Bearings in it didn't look too hot either (showing copper) but that's unrelated to the varnish. But again, the car "ran fine", if you didn't take it to the track, you'd never have known it had lost enough power to be so much slower than another stock example and his is not the only example I can think of, another good friend of mine had a very similar experience, car was a lighter notch and I think the best trap he ever got out of it was 92. Normally was running 87-88Mph. Engine had probably half the mileage on it that mine did but wasn't as well maintained.