Well, I guess it is good that we got the cardiogram for Bonus Cat. And that means we have bad news.
His heart is fairly deformed, it turns out, which, as you can imagine, leads to all sorts of problems. I had to ask the vet to repeat the beginning of the list in the middle of it because I had forgotten it. Among the problems is that he has a blood pressure of 210 over something (which is partly accounted for by being stressed out by the vet, but nowhere near all of it), and this could be why he yells at night. We assumed that it was because he was bored.
The possible out that we are hoping for is that the whole heart situation is stable, despite the weirdness. If his echocardiogram in six months is the same, then maybe he has found a way. I did some pull-ups about this, and it occurs to me that his heart has always been like this, at least somewhat.
He has always been a weird cat. He used to do a lot of sudden flips, run in hot weather until he was panting, and has a hole in his ribcage. He is, as Katt says, Slightly Irregular. Now we know the irregularity extends to his heart.