The slot a GPU is attached to shouldn't make much difference at all, as long as the latency is low, and the card has enough memory.
Using a high speed interface for a video card is a low-end solution. It means you didn't put enough memory on the card. Using system memory slows things down, either way. If you use a slow interface, it slows it down even more.
GPUs get commands to do work, which are tiny. Provided they get them quickly (i.e. low latency), and work with their own memory, interface bandwidth is essentially irrelevant, for a single card. The downside is applications that use the GPU for things residing in system memory, to assist the CPU.
So, they'd have to sell these things with lots of memory if they don't have great bandwidth, and they'd probably be a bit more expensive, but, then, laptops always tend to cost more, and have more expensive parts. They'd probably also want to avoid apps that work with system memory, when possible.