It's all about the amount of VRAM...You probably have the link, but the extensive manual explains it well:
http://www.x-plane.org
It's worth joining the site for downloads too, but it works best with more video ram...It will run with 512, but if you are used to running it on your iMac, then I think you will be less than happy with it on the MBP. I don't use mine anymore, just stick to the iMac....3.4GHZ I7 16GB RAM 2GBVRAM...and the manual says it will take advantage of much more if you have it. The site is a great resource for free aircraft, scenery etc., too.