Your current MBP gets a Multi-Core Score of 11721 as it is a 4 core CPU. both the new MBA (Current Max CPU) coming in at 6841 or the 13 MBP (current Max CPU) coming in at 7366 only have a dual core CPU. Having said that if your current MBP 15 has the 650m, not only will your current CPU be faster than either of these iGPU's, but the CPU power is also much better as well as it has two extra cores.
Now, If your 15 MBP only has the Intel 4000 the MBA or MBP current may actually perform faster for games as the MBA has a Intel 6000 and MBP has a 6100. Even the new 15in with iGPU has a 5200 pro thats the best right now but wont beat a dGPU. Otherwise the only jump you will get is getting another 15in with the dGPU of either the new one with the x370m or getting a refurb/used model with the 750m but it would be a smaller upgrade.
https://browser.primatelabs.com/mac-benchmarks