Quite a simple one really... How loud is it?
I bought the cat back of Jimmy and rather than cut the standard up I thought of getting a decat. If the catback is too loud on it's own then I won't be having it on the car very long

As its a res cat back it shouldn't be to loud,and decat on a standard exhaust would be pointless without a cat back,if you have no map you would loose bhp more then anything

Sorry not very clear, I'd run the decat and Piper catback then come MOT time slide the originals back on
Biggest problem is I leave and arrive home at "inconvenient" hours for the sleeping neighbours

Really? Whys that?

I found my decat standard system made my car much more responsive...

When I changed from the standard cat to sport cat with the Piper res, the noise from the back box didn't increase a lot. The increased roar came from the cat itself so it was under the car rather than at the back, so possibly not that affected by the type of back box.

I think Watkja used to have a decat Piper.

PS: I'm not sure if the Piper de-cat fits straight on, so check first that it will fit onto your current manifold/flexi. They sell silenced de-cats too.

Not sure why mate but over the years I've seen people put de cats on and loose power from dyno results you may want to do a quick google search and see what comes up

http://www.nsxprime.com/nsx-faq/exhaust-theory/ A nice article about exhausts that try to explain what an exhaust does and why

Well I'm presuming the catback is resonated as it's got a silencer box on the outlet pipe to the backbox silencer itself. Kind of bought it on a whim lol
I'm not too worried about power output, it'd be nice but the ST's aren't exactly the fastest things on the road.

Yeah the cat back is resonated... what I meant is that Piper also sell de-cats with a resonator in them to quieten it down http://www.piperexhausts.co.uk/piperstore/product_info.php?cPath=26_236_453&products_id=3547

I reckon you could just fit the new cat back without spending any more on de-cats etc, and then it'll be easy to just re-fit the standard cat back if for some reason you decide you don't like it.

Yep, i had Piper decat and non res. Was Really really loud!!!

Well hopefully resonated will be a little quieter lol
I think I'm going to cut the standard system and whilst it's off I'll weld a sleeve in so I've only got one new joint rather than wobbly 2 joint jobby

Whats the piping size on the decay pipe? 2.25" is the usual size for them kits isn't it? So if your keeping your standard kit your going to run into issues as that is 2"?