I'm running GTA V on a 21.5" iMac with the Nvidia GeForce GT 750m which is almost identical to that of the maxed out 2014 15" MacBook Pro. I am able to run the game at 900p with settings ranging from normal to very high, with FXAA (no MSAA), VSYNC off, 8X AA... At around 60fps on average. On foot, I can see myself hitting 80+fps, and when driving in heavy traffic the lowest it goes to is around 45fps.
I've spent a lot of time on tweeting the settings to get the best perfromace and graphics at the same time, just ask if you'd like the settings I use to get that level of performance 🙂