Sly, a "super" high flow is basically a straight thru pipe in a cat housing with no internals - sounds like that's where you started b4 u put on the high flow cat .
If the engine needs to breathe, u will usually see about 10kw difference between a straight thru and a high flow
Don't know much about RB25's, but u could be right that 2530 is too small - hence exhaust flow problems
lets face it, 2x 2530's on RB26 is magic for 300 - 350 kw applications
good luck with the continuing developments - sounds like u need those bigger injectors :-)