Well, the Japspeed being super high quality could be miss aligning the rest of the system.
Motordyne make the kit to fit like oem, so in order to see if the exhaust is at fault or the decats we shall need to put oem cats on to see. If it fits its clearly the Japspeed decats. If it doesn't fit we know its the exhaust.
Time, effort, money.....
All in the love of our cars!