well, I have been there and done that - and I experienced it first hand because I do the tuning and mech work.
I had a Walboro that couldn't keep up at 300rwkw, and must have dropped pressure a little bit. When I had the car on the dyno, the AFR's leaned to 12.7:1 but that didn't matter as I have water methanol injection.
Anyway, my car was tuned for a pump that was delivering below 42psi at high load.
I put a Nismo 275lph pump in, and it now had enough grunt to delivery a full 42psi to the fuel pressure regulator.
AFR's dropped to 10.7:1. So the pump ust have been dropping a fair whack below 42psi.
Just remember, the fuel pressure regulator ONLY keeps the pressure at 42psi if the pump is able to deliver more. If the pump can't deliver more, the FPR can't do its job. All it does is returns any fuel over 42psi back to the tank.
If all that makes sense. You don't need a new regulator. You need a pump that can keep up to your demands.