There are A couple of differences with a and double barrelled phase 1 cat
I have the ktec supersports system (no mid silencer 2.5" straight through) with a NW performance decat and if I'm being honest it's stupidly f**king loud in the car, if you do any motorway driving (long distance) don't get one! I did 180miles yesterday and want to get rid of the f**ker. But then if you just go around town, b-road blasts ect ect (which is wot I mainly do they are not to intrusive)then get one coz through the rev range I love how raspy they are.
Another thing that I have defiantly noticed is at about 3.5k-4k rpm the car revs a little quicker but and it's a big but it has not got the same pick up from idle to 3.5k rpm it feels much more delayed.
It is a pig to fit on the phase 1 172 as well pay a garage to do it unless you have hads the size of a ten year old lol
Im pretty sure it increased my mpg as well, it's just so fricken noisy lol