O ya, clogged cat is very possible with those symptoms.

 

i didnt mess around with installing the head, i had a local shop do it....

 

I had one of the Cats checked, and it was fine didn't bother with the other since the first was clean...how much does it cost (aprox.) for a compression test. And how much fuel pressure should there be. I changed the fuel filter a lil while back and there was a slight but noticable improvement in performance...

 

OK, Im no expert, but I'll throw in my 2 cents, and diagnosis.

A lot of the things you mentioned, bad mileage, no power, backfires, sounds like a overly rich fuel mixture, which the first thing I would look at is a bad maf.

Its easy to test for this, just unhook the maf and see if the car runs better. if so, probably a bad maf.
But, for this, or a bad plug, or misfire, or o2 sensor, just about anything, should throw codes. So I am not sure what to tell you there. Use the vag-com directory and see if you can get someone with a vag com to help you out, monitor air-fuel, see if there are any codes.
If still no codes, I would: pull the plugs and see what they look like, that can give you direction. Also, I would reset the ECU to clear up the possibility of it malfunctioning, at least maybe this could kick the codes back in.
So, unhook maf, check the plugs, reset the ecu (pull neg bat. cable for 5 min-you'll need radio code) and try to get it vagged, and report back here all your findings.

Compression is not hard to do yourself, get a haynes manual and it has the procedure in it. that will rule out bad head gasket, or bent valves, or warped head. Have you noticed anything stange in the oil, cloudiness or white in the oil? That would signal bad head gasket.

Good luck, keep us posted, there area lot of folks here that can help you figure this out, most of the time someone has had the same problem before at one time.

 

compression test, cylinder leakdown test, pull plugs and inspect, maf test and somehow inspect the catalytic converter. And if you need new heads which i doubt if its running for 4000 miles you will need 2 obviously because of the v design vs. 1 head on the 1.8t, so it would be more expensive. these tests should guide you on the right path. I would also do a injector test for the hell of it to eliminate the fuel issue which is to pull each injector and see if the idle changes.

 

Could you please elaborate on the injector test. How exactly do you do it?