As above however to answer your question. In short, yes they will lose Hp like any aged car.
I'd have to disagree with that statement i'm afraid.
Providing it's in good condition, and well looked after (serviced as it should be an all vacuum lines in order), I'd expect a 200k mile AGU to still be making 150bhp or more.
These engines are remarkably good at covering high mileages, my original AGU engine had done over 216 000 miles when i removed it, with over half that spent at 230bhp, and the final year spent at 300bhp without issue.