I was going to say i bet it was the sensors conflicting. I think with the high flow cats you need to get a MIL eliminator. This basically bypasses one of the sensors i think and tricks it into thinking that both are working ok. I had the same issue on my old MG. Maybe its the same thing with these?
Shouldnt need to with hi-flows only with decats.
Get it into a dealer & see what codes it throws up or try the pedal procedure yourself.
You could also buy an OBDII reader, comes in handy sometimes