FiTech EFI system

58 psi seems to be the regulated line pressure.
Pressure in the return should be minimal, if it gets much above half the line pressure, you can run into problems - but the line pressure is the real issue. Putting a fuel pressure gage on it would help determine if you really do need a bigger/better return.

I'm using the original feed as a return also with a 255lph pump mounted in-tank. WOT is fine for me.

WOT should be the biggest demand for fuel, so the return shouldn't be the issue. The return is most heavily tapped at idle/low flow times. How full is your tank? Possibly fuel starvation due to fuel slosh and an in-line pump? in-line pumps are notorious for starvation when the fuel sloshes..