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..