For an EFI application, are there advantages to where its mounted, up front with engine, or back in the fuel cell area?

Seems in the back would require a high pressure pre regulator "return line", with a short bypass hose to tank, and in the front would require a large "low restriction" post regulator return line.

Any real difference performance wise?

I'm leaning towards a rear regulator.

I think it depends on how much power you are trying to feed. For a normal street car with 400 or 500 hp I'd copy the OEM setup. You can buy a Corvette regulator which mounts on the frame rail near the pump. They are only about $30. It also depends on the EFI system. The Holley Sniper has a built in regulator so you might as well use it. For port injection you might contact the vendor to see what they say. Some vendors might want you return style setup while other vendors are fine with dead head. Holley recommended a dead head system on my Duster but I've had multiple EFI guys tell me that it won't work.

I put mine up front as what Holley said for my HP multi point
600HP system.. it will handle higher HP

I think that Holley has changed their recommendation since you installed that unit on your car. Holley came out with a line of fuel cell pumps and trunk mounted regulators and now that is what they recommend.