Q: memory bumbs up RAM upgrade..

When the GPU shares system RAM (as the GPU integrated into the CPU does), adding system RAM does bump up the memory available to the GPU.  But only just a bit.

The GPU is still part of the CPU, and heat from both the GPU and CPU combine to require occasional slowdowns to dissipate heat.

This Apple Note will answer your question for your system.

"Could" being the important word.  Remeber more video RAM does not always produce dramaitc results.