There is more to it. Bandwidth is one of the top things there is to a GPU (besides shaders and fab process). The faster you move things the better. Laptop GPUs are more powerful than what people think, but they are severely bottlenecked by low memory or low bus rate. A solution has been what you described, up the DDR speed from GDDR3 to GDDR5. Works, but it is not the solution. Just look at ATI's and nVidia's current gen cards, all use 256-bit (performance/enthusiast) and 128-bit (consumer). However, the better performance cards are always those with ample bandwidth to the VRAM.
Why? Because they have to move massive amounts of data to/from the VRAM. You can look at many results and will find that fact holds true thru all video cards. Moreover, adding more VRAM will only solve the problem if you are doing something in an image editor like Photoshop. In games, having 2GB is mostly unneeded currently as current cards haven't even tapped 1GB potential. nVidia just broke that 1GB barrier with Fermi based GTX 480 and GTX 470 at 1.2GB configurations; test show that all the large .2GB really does it keep minimum frame rates up, but doesn't add any potential.Test was on intensive game like RE5, Batman, and Far Cry.
In Photoshop, the amount of VRAM is what is important, in games and other applications that require the GPU to push data fast, it's the bus width and data rate that matter.
Apple's laptops are not oriented at games, so they don't need large VRAM. People forget that. Apple has its OS X for media stuff, like pictures thru iPhoto or your favorite image editor, movies (which are more CPU intensive) and audio (Logic Pro, and other audio tools). Sure Apple pushes games, but ideally, MacBook Pros are not for games, that's something people have added by using Bootcamp and putting Windows in there.
