I agree with Walter. Unless you are using Ray Tracing in After Effects, the GPU is not important.
Even if you are, there are laptops with good GPU’s that won’t require an external thunderbolt connection.
Even Apple’s new MacBook Pro Retina has the nVidia 750M with 2GB of VRAM. Certainly not the top of its class but sufficient for Ray Tracing.
If you need more then you may want to consider a Windows-based laptop or shipping a workstation as Walter suggests.
CPU speed, number of cores, disk speed and available RAM are the most important factors for render speed. Not GPU Â … currently, and that may change.