8 vs 16GB RAM:
For gaming this will make no difference at all.
Today the real difference is going from 2->4GB, some from 4->6GB and not so much over 6GB. Anything over 8GB is a waste for gaming, but can be nice for other stuff.
512MB vs 1024MB VRAM:
Games are usually limited by how fast the GPU is and not how much VRAM it has.
To make a game use more than 512MB VRAM on a 1440x900 screen you need to max out textures and also add some anti aliasing and other effects.
Usually the 6750M will struggle with settings that needs more than 512 MB VRAM anyways and most of the time the extra VRAM will be wasted.
However if you can live with lower FPS with better graphics more VRAM will help. There are of course some games that are very VRAM hungry, like some MMOs like SWTOR, other games like BF3 do also want tons of VRAM, but turning down settings will make it use less VRAM and you really wanna do that to make the game run nicely on a 6750M anyways.
TL;DR: If you like to play at 60 FPS and medium/high settings then the 512MB will be as good as the 1024MB one, if you like to play at everything maxed with 25 FPS then the 1024MB version might be worth it.
The games you mentioned:
I have only played Borderlands (but windows version), and Diablo 3.
I finished Borderlands before the mac version was out on my 2010 MBP with GT330M, the 6750 is about 50% faster and should run the game very nicely.
As far as I've heard the mac version is pretty well optimized and should play very well on the 6750M.
Diablo 3 is still in beta, it ran pretty much perfect in OSX on my machine, and I've heard reports that people with much less power than the 2011MBP has ran it very well.
Mafia 2 should run well on a 6750M, but the reviews on App Store tells a different story.
http://itunes.apple.com/us/app/mafia-ii-directors-cut/id481672738?mt=12
They might not be accurate tho, as the game should run very well on a 6750M if it's fairly well optimized. (Windows requirements are 8600GT/ATI HD2600, and the 6750M should be much faster than those.)
Bioshock 2, I have no idea, i think feral have made it compatible with intel hd, the 6750M is tons faster than intel hd.