With respect to modern games in Windows like Crysis 3, Tomb Raider 2013, etc. I would say that as long as your happy with Medium settings at 1080p or lower then you'll be happy with the 650M in the 15 inch Retina. I think in maybe 3-4 years as long as you're willing to drop the resolution down to 1440x900 or 1280x800 with medium settings you'll still be OK. That's just a scientific wild a** guess though.
People here think the Haswell rMBP 15 is going to be a huge leap in gaming performance. Nope. The 750M is just a clockspeed bump with minor improvements. The iGPU will be the biggest improvement and may spur Apple on to do a 'cheaper' rMBP 15 version with only a iGPU as a more affordable 15" option. But for gaming, the Haswell rMBP will be at best 10% or more so faster. As the majority of modern games are GPU limited on the current rMBP. The next big bump for portable dGPU's will be with the next die shrink with the '850M' generation.