I plan on purchasing the new iMac when it comes out in a few weeks, but I'm not sure that I'm going to like OS X.
If I install Parallels and use Windows 8 on the iMac, would there be any sort of drop in performance, as compared to using Windows on a regular PC?
And would there be any other cons?
It depends on which performance you are talking about.
Please check my benchmarks there:
http://forum.parallels.com/showthread.php?t=208365
At my latest benchmark, CPU performance was just 2%-5% worse, while GPU performance was 26% worse.
Then, Parallels Desktop 8 came out, with full DirectX 10 support.
I believe, the performance difference (both CPU and GPU) is really small now.
The only trick there is that you would need to upgrade your RAM,
because virtual machine eats half of your RAM. So, for the best experience,
it would be great to double your RAM before using Parallels.
The whole thing is very convenient to use: you could switch between Mac and Windows
with one simple gesture, in just a second; no need to reboot computer and wait - like with Bootcamp.
P.S. At the moment, gaming performance in Windows 8 is a bit worse than in Windows 7.
If you care much about gaming, it is better to stick with Windows 7, until Windows 8 gets more mature.