i have just had a new back box and centre decat pipe should i do a ecu reset or will it sort it's self out over time is it usual for it to pop a little say letting off throttle in third it also seems not as sharp low revs but a lot better mid range is this the norm

Just decatted mine and its exactly the same m8. That pop is most likely a flame of unburnt fuel out your tail pipe. Nothing to worry about.