Unfortunately, straight pipes will give you more flow, but the MAF and ecu will be miscommunicating. The loss of your O2 sensor (bank 1) at the cat will be missing (bank 2 doesn't really matter since it's after the cat). The missing O2 sensor will give you erroneous readings (you'll get a CEL, too) and your engine will suffer. I think piggie pipes use a cat, but I could be wrong.

 

The atp downpipe has the o2 bung in it

Ive read of alot of 1.8t downpipe users using a kind of spacer to back the rear o2 sensor out of the direct exhaust flow effectively avoiding a cel, has been used for weak cats too