I've had an aftermarket downpipe for over 10 years without a tune.
 

To make a long story short I bought my 2008 MS3 in 2009 used with only 11k miles from a dealer. They said warranty was intact and I assumed it was stock.

Fast forward a couple of years and a CEL popped up about catalyst below efficiency. Dealer replaced O2 sensors under warranty. CEL came back months later and dealer replaced the O2 sensors again and reflashed the ECU. It was fine again for maybe another year and then the warranty ran out. After it came back a 3rd time I complained that they should still fix it because they didn't fix the problem the previous time. They investigated further and found that I had an aftermarket downpipe/cat. I guess under certain conditions the aftermarket cat doesn't work as well because it's freer flowing and trips a CEL.

I decided to just clear the CEL and not worry about it since it was fairly infrequent and the car drove fine. I've owned the car for about 11 years now and started having (new) problems. I found out that I've been overboosting at around 17.5psi. After searching the forum I discovered that I am overboosting because of the aftermarket downpipe and that I should have had a tune. I'm going to assume that it's not tuned because the car was otherwise stock and also the dealer reflashed the ECU at least once. Also, I've always gotten worse gas mileage than other people on the forum.

I'm wondering now what my options are. I was able to see the downpipe was stamped with Turbo XS but I don't know how much of the exhaust was replaced. I'm also not sure if I've done any engine damage. I've got 96k miles on the engine.

Should I try to replace the downpipe with stock or try to get it tuned? Or at this point should I just leave things alone?

i would recommend getting a hpfp (or at least check to see if you have one installed aready)

do you have an accessport to monitor the boost?

maybe get a cheap e tune... full tune would probably be best.. post some pics of the engine bay and maybe we can see if there is any other non-stock parts

You might want to look at my mods. You need a hpfp upgrade and a tune.

Believe it or not, on the stock K04, the often derided Hypertech tuner maxes out the K04 or pretty close to it with safe AFRs and controlled boost up to 18 psi with taper to a safe 14-15 psi while maintaining aggressive timing advance. And, yes, that is with a catless 3" downpipe and racepipe and an aftermarket CAI or SRI, if on their list of intakes for their MAF calibration tables.

With your mods it will likely get you a safe 300 whp once you upgrade the fuel pump.

If you can pick up a used Hypertech Max Energy tuner, that may be your best option and will get you 300 whp on pump 93 octane. New is about $400. If buying used, it must be unmarried from the ECU it was programmed to. Otherwise, it is a brick. Same with other tuners like Cobb AP, if bought used.

It will NOT work properly, however, if you use a different turbo, a larger diameter MAF housing or if you try to use ethanol or other fuel than 91-93 octane pump gas.

Either the plug-and-play Hypertech programmer,mor get a Versatune or Cobb AP tune compatible with your mods. The AP off the shelf maps are not very good.