I think you may be confusing the RAM allocation for the Intel IGP as being applied to the dGPU as well.
Allowing a game to run over into system RAM is likely to cause as much or worse performance degradation as just keeping the 2GB VRAM limit in place.
Running out of system RAM because you're using it for the dGPU or IGP is worse again, because now you've taken a HUGE step down to system storage (hopefully SSD and not HDD)
The fact is, your system as a performance limitation based on the available hardware. Reduce game quality settings first, then resolution until you get acceptable frame rates.