the way graphic cards work is like this
650 < 750 < 850
670 < 770 < 870
680 < 780 < 880
the x50 cards are low end cards, you can get an msi gaming laptop with a 970m which is 2nd best tier for around 1800$ which is thinner and more storage than rmbp.
apple upgraded their 650m laptops to 750m rather than giving apple customers a better card, the upgrades from 650 to 750 to 850 to 950 are not much... sometimes there's barely any difference.
you can check wikipedia geforce card specs... the max gigaflops the 750m makes is around 740 gigaflops
https://en.wikipedia.org/wiki/List_of_Nvidia_graphics_processing_units
meanwhile the 870m has 2500+ gigaflops
970m is around 2500+ aswell
so if you want an apple computer, not only will your OS run a crappier version of whatever game, but it will also run on low graphics in low resolutions.
i used to buy apple laptops, but i couldn't take it anymore... i went with msi which is cheap and awesome, msi gaming laptops are better than alienware and asus rog.
i really miss OSX... but i can't bare having 750m, its 3 times worse than the card i have now 870m and i only paid 1850$ for this laptop... while the rmbp is 2650$ (canadian money)
i'm just being honest here, if you want to play games... dont bother getting the current model... hopefully the next model will be better... but if its a 850m or 950m ... its still ****. (i dont think 950m exists as of today)
ps: if you got that amount of money, spend it on 5K imac
https://en.wikipedia.org/wiki/List_of_AMD_graphics_processing_units#Radeon_Rx_M2xx_Series
the graphic card in the 5k is almost identical to the 970m, except 5K resolution requires more power, you'll have to downgrade your resolution in gaming to 1080p