GPU Upgrade for 2010 21.5” iMac

I have a mid 2010 iMac 21.5” with 12gb of ram, I am very happy with CPU performance but the GPU performance is extremely lackluster (It currently has a ATI Radeon HD 4670 with 256mb of vram).

As it is a 2010 model it has no thunderbolt port so an eGPU isn’t an option and finding upgrade cards that will fit in it is extremely hard and expensive and they are very underpowered.

So I had a crazy idea to take my iMac apart, put an extension of the slot in place of my current GPU, cut a hole in the back of my iMac, run the extension through the hole and plug a powerful standard desktop GPU into it with an adapter (if one exists), the “external” GPU would be powered with a different PSU as the slot cannot provide that kind of power.

Could this actually work or would there be some firmware issues?

Thanks