FiTech EFI system

Gotcha, it's not so much an absolute size issue, but a size difference.

I'd pressure test the supply/return. If the return is too small, the supply won't be consistent or will be much higher at lower fuel demand modes (idle, light cruise).

WOT is still where I wouldn't expect to see the issue, unless your supply pressure is so great (due to the return) that the learning is thrown off and then WOT goes off the VE tables when the pressure returns to normal.

You could likely use regular hose and run it back to the tank, but if you get any kind of resistance in that return or a kink somewhere, it could pop. A pressure test is a lot less risky, IMO, but a temporary/test return *could* work.