I had Pypes LT, Pypes O/R X & Catted X, Pypes M80 MMS on my '08 for 7 years. When I went Procharged, I swapped to MBRP (street series) catback and absolutely hated it. Sounded like an old farm tractor. Bought the Magnaflow (not the glasspack style) catback and while it was a tad quiet at idle, it was race car at WOT and had great tone at cruise loads with minimal drone. Only reason I didn't run the M80 MMS was the volume, they're terribly loud. But my cammed NA setup sounded like pure sex singing through the Pypes setup.
I think the only issue I had with the LT install was a few of the open flanged holes (the side of the flange that's closest to the bottom of the heads) didn't quite line up perfect and I didn't want there to be tension on the header. I used a grinder with 1/8th" disc and lightly opened up the fork a little wider. They went on perfect after that.
I also used Percy Seal-for-good exhaust gaskets. In the 10 years I owned the car with Pypes LT's, I never had an exhaust leak at the headers.
I think it also makes a difference; after I ran a couple heat cycles on the engine, I went back and checked to see if any nuts had backed off, I got a 1/4 turn off a few. Everything was red-loctite.
When installing the headers, I recommend bolting up before stud placement. Do the top flange holes first. If they line up, it's the open holes on the bottom side that need adjustment.
For double the price, Kooks had better be spot on in fit 'n finish. For the bargain Pypes was, I didn't mind filing on a few flange holes to acheive fitment. If there had been other issues or no work around, put the hammer down on customer service 'til it's right.
I just can't stand people knocking "china knock-offs" and then chest thump their Manley internals. Where do you think those are made?