A lot of Apple's attempt to keep a larger profit margins is by building a limited amount of parts. The mid model uses the M390 while top uses the 395 or 395X, which has different VRAM configurations. You can justify this by your level of graphics processing demand and graphics texture use demand. If you want to compare them, you can reference this link:
http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html
There is a learning curve to go with how this is used, but unless you're running really large graphics loading games, the 390 will do ok.
If you really want a high end graphics card, I would wait this cycle due to the die shrink and the control of the temp in regards to the voltage increase on the new nVidia and ATI cards. In real world, I think we could get up to 1.5x the performance of the equivalent cards due to this upgrade, but both companies are claiming closer to 2x. Again, this is only if you are a gamer, if not, the current models are excellent.