I have an ATP down pipe with no cat mated to standard cat back system.
I think it sounds stunning and in conjuction with stage 2 remap she now makes 160kw's @ the front wheels.
Found out on Friday.
Still running standard intake and filter.
I've got a similar setup to you (minus the stage increase) whose map was that with and what dyno?
I would say most would agree that a 3" downpipe and decat gives the best performance if you can be bothered to swap the cat at MOT time. The spool is a lot quicker and low down torque is better too. Failing that you could try a 100cel cat and risk it passing emissions or a 200cel for definite pass.
I'm running APR Stage 2.
And the Dyno company in New Zealand Called torque performance. I have dyno'd all my cars for the past 8 years with them.
They are pretty accurate too. But it's not a hub dyno.
I don't bother swapping the CAt when it comes to our WOF (your MOT) time. As they are not yet that bitchy here. However if we cannot install Adaptors for porsche wheels. We can install remap or coil-over suspension, but it has to be certified. At a cost of $510. If any changes are made. IE height on the coil-overs. You have to get it recerted and again another $510 down the drain.