GTA IV's performance GTX 485M Vs my GTX 680M is not much different. Sure, I can get slightly smoother performance at higher settings but the frame rates are still from 28-45+ fps, in the air is is 80 fps, some areas are 50+ fps but none is ever consistent and there are always the occasional stutters.
This game is very poorly optimized and the only thing that will make much of a difference is a very powerful Quad Core clocked at a much higher rate. GPU's make a minimal noticeable difference on this one unless you go from low to high spec i.e GTX 285M to GTX 485M.
In comparison, I can play SR3 maxed at 60 fps with no stutters at all. The GTA IV Enb Mod will not make matters any better. However, if GTA IV was straight to the PC, we would most likely not have any of these issues.
Only if a game is heavily GPU dependent (like most), would we see better performance in this new generation.