I think I may be able to answer this only since my laptop is a T60p that I use with similar programs for mathematics (texturing for presentation.) Really it comes down to the maturity of the drivers, especially with ATI/AMD. When I got my laptop the FireGL V5250 ran fast and appropriate right away with the initial drivers, which was unlike my desktop with a x800 pro. When that card first came out the drivers I used were the standard "gaming" drivers and it wasn't till the FireGL X3 (workstation x800pro) drivers had came out that I got decent performance in non-gaming 3D applications. So i'd give your 4870 a little time, especially wait until the FirePro 3D V8700's(4870) and V7700's(4850) are completely supported in Maya.
Nvidia does their drivers in different batches... not sure if your performance has changed with your 8800. But from what I understand the NV Team's workstation department and gaming department are more integrated than ATI's. This has been good and bad in the past... ATI usually has better initial support and once it works, they're done. NV on the otherhand is a little more random depending on the application, they tend to "tweak" more. I can explain more if you'd like.. but i'd rather not bore you with the politics of gaming/workstation cards