I run A3 on a 2009 15" MBP unibody (base model) without any issues. I have had photos with up to 15 layers going (brushes including retouch, burn, enhance, crop, B&W etc) and don't have any problems. Occassional slow down when applying a new brush but it's usually because I'm at like 20 layers by that point and just run out of memory.