cheeseypebbles I'm having the same issues. I couldn't find help anywhere on this issue but here... Some people have said to turn off Steam's in-game overlay. Did that but it still didn't work. It still lags. I've watched youtube clips and i'm in awe of how smooth it looks but on my Mac its just so lag. I also have the same specs as you do...
Processor 3.2 GHz Intel Core i5
Memory 16 GB 1600 MHz DDR3
Graphics NVIDIA GeForce GT 755M 1024 MB
Software OS X Yosemite 10.10.2
Have you found a way to fix this?