Maybe I'm lucky, but I don't seem to have any real issues with SR2 PC (I've played it a few hours so far). My CPU is Intel Core i7-3610QM 2.30GHz, playing at 1920x1080 with almost everything at max.
I did see in e.g. Youtube some people having severe framerate issues while driving, but I haven't seen that either. Maybe I'll still give the unofficial patches a try, just to see if they matter to me.
EDIT: Ok if it may affect some timed missions, that is certainly bad.
EDIT2: Reading the Steam forum discussion, maybe this is the reason I don't see the speed issues then:
How can I fix Saints Row 2 running too fast at 1.5x speed?
IMPORTANT! Many players have reported that the speedup bug is now fixed with Windows 8, and Minimaul confirmed it as well on his test box! DO NOT USE POWERTOOLS OR CHEAT ENGINE IF YOU HAVE WINDOWS 8!
The speedup bug is something that is an inherent problem with Windows 7, though in rare cases it has been reported to happen on Vista. The speedup happens with CPUs that do not run at exactly 3.2Ghz frequency (the Xbox 360 cpu speed.) The further your cpu is away from 3.2Ghz (both higher and lower) the faster it will run.
Well, as a matter of fact, I am indeedy playing it in Windows 8. Interesting, it works better on Win8...? Maybe I'll try it on Windows 7 too, just to see what people are complaining about.