In my Lenovo Y500 with a GT 750M I can run Battlefield 4 on all ULTRA (100% resolution no AA)...The R9 275M in the Lenovo Y40 is slightly faster than my GT 750M, paired with an i7 it should perform about the same as my laptop.
The NVIDIA GeForce GTX 860M in the Lenovo Y50 is quite a bit faster than my GPU and paired with an i7 should offer a budget desktop level of performance.
In terms of any GPU on any laptop, I would go with NVIDIA every time!
Anything GTX 760+ (680M would be good too, just avoid anything below the 675MX) So, a GTX 850M would be phenomenal in my honest opinion, especially with the Lenovo Y50 as an example that GPU would come at a very sweet price--point for you, we just have to find a model with it!