He’s already logging fuel pressure which would determine if the fuel system is up to the task or not(PLEASE correct me if I’m wrong!). He is running 58psi base fuel pressure on the old injectors and the new injectors. He’s drawing fuel from a welded on sump that may have it’s weakness’s, but only at very low fuel levels or when cornering. He’s faced fuel delivery issues in the past and I just don’t think that’s what’s happening here.
Subcom, in the past when I ran two pumps I triggered the second off a Hobbs switch at 10psi boost. However, I was running a stock fuel pressure regulator at the time that couldn’t bypass enough fuel. Hopefully going off throttle% works for you, but if not try triggering from boost pressure.