Hey Dude,
Im using a Retina MacBook Pro 15" with parallels- and I am very happy with it.
regarding your questions:
1. Parallels has strong advantages:
1.1 coherence
you can run parallels in so called coherence mode, i.e. run windows app in MAC window - this way you can continue to work on your MAC without having your display blocked by windows. e.g. I am writing these lines in safari while having an IPoker window open 
1.2 security
you can fully isolate the windows machine from your MAC at runtime, which is very important, in case you get attacked your business stuff is still safe on your MAC. ... poker environments are not as secure as they seem ... ♠_wink:
1.3 backup
backing up a 100gb VM into a zip file takes 20 minutes, restoring from zip file 8 mins
incredible performance. If something goes wrong, you're back and running in no time.
2. specs.
I have allocated 12 gb for the poker vm, and 2 cpus - reason: the postgres database becomes extremely resource intensive after a while. It is better to give him an extra cpu and 8 gb RAM, this way you have no delays multi tabling on the vm.
3. PT4/Pokerstars performance
PT4 for MAC works fine. I do however run everything in windows vm. From the lines above, I think it is clear why.
4. useful tips
- You will want to take 15 inch with the strongest graphic chip.This way you are able to exploit the full resolution. i.e. 2880x1800 + 3840x2400 on external display.
- assign separate MAC address to vm and run on separate IP (shared mode gives delays sometimes)
Good Luck!