I am running a catless DP/RP combo. It is the TurboXS, which I consider to be the best bang for the buck if you can run catless in your state.
This is a far more complex issue than OP's simple questions. Probably why there have not been many responses. Easy to ask questions, hard to answer them when there are multiple and highly technical issues involved. I will offer an opinion, but I ask OP to also use the search feature and read, read, read, study, study, study, because there are no simple answers to this mod decision, IMHO. There are many factors that have to be taken into account in combining this mod with other mods in a safe way that contributes to a good running car.
1. AFR's will be a bit leaner but will still be pig rich and safe if the only other mods are a good intake. Start making other mods and you could get in trouble.
2. MPG is purely a function of your right foot. Don't install a DP/RP expecting to either gain or lose mileage. I can't tell any difference.
3. You must have a good boost gauge and must pay attention to it. I see from another thread that OP is working on that. You also need a way to at least spot check AFR's under load with something you can attach to your OBD2 port, just to be on the safe side.
4. I do not get boost creep. None. Nada. Zip. I do get very quick momentry spikes to 20-21 and then the boost settles immediately back to target. More about target in next point.
5. In my experience, with just an intake mod and a catless dp/rp in my moderate weather climate, the target ends up being as much as two psi higher, depending on gear and load. It will typically hold at 16-17 psi rock solid. Under some conditions I see a steady 18 psi. This is about a 2 psi gain over stock.
6. I am running the stock BPV. This may or may not be important, for reasons I will explain. I think the stock BPV gets unnecessarily criticized. Mine does not leak, or if it does leak it is a very modest controlled leak. Let me explain what I mean. Some guys running aftermarket BPV's and dp/rp's are running into boost creep where after the spike and settle down, the boost then starts climbing back up above the target value. This may be a function of operating under different temp/humidity conditions than I have. This may be that aftermarket BPV's and the stock BPV differ in some way such as that the stock bpv may have its own built in "boost leak" above 18 psi. I really don't know. This is complex and poorly understood, at least by me. What I can say is that with the stock BPV I am holding rock solid at 17 psi, +/1 one pound under load and have no boost creep.
7. Boost creep. If you do get this - climbing boost above 17-18 after the initial spike and settle, this is dangerous. There is no easy fix for this. To a certain extent, AP's tuning can compensate a bit. Maybe the stock BPV is doing the same thing for me. Dunno for sure. The only proper fix is to remove the turbo and have the wastegate flapper port enlarged so that more exhaust gasses can bleed off. The reasons for this are technical and would take some time to explain.
8. You will most likely throw check engine lights regarding the conditions of the cats (which will be gone). Either live with that or do the "diode fix" (beyond the scope of this reply.) The spark plug extender fix did not work for me.
9. You are likely to encounter fuel cut and/or boost cut in colder temps. I view this as a good safety feature of the stock ECU. Aftermarket ECU tunes that eliminate these safety features while running boost levels above stock are not a good thing, IMHO. Others may differ.
10. Speaking of fuel, there are fuel pump issues which may emerge. The fuel pump is marginal on this car to begin with. When you increase fuel demands with both a good intake and by eliminating the extremely restrictive stock DP/RP and their cats, fuel demands increase with the increased power. This can cause your fuel pump to not be able to keep up with demand, resulting in a drop of fuel pressure. Your ECU will probably intercede to keep AFR's safe at the expense of performance. One member here has documented this phenomenon well - Darth Vader. My pump seems to be holding for the time period.
11. Installation: It is a b**** and not easily undone. Warranty will be shot. You do have to be very patient and reasonably competent with hand tools. You will need a variety of different length extensions and "wiggler" sockets. I have a shop, lift and just about every hand and power tool you might want. But because this is the exhaust system and exhaust nuts, bolts and 02 sensors tend to freeze up over time from the extreme variations in heat, it is easy to strip out a stud, bolt or nut. So I contracted out the install to a local performance exhaust shop. It took two guys four hours working both inside the hood and from under the car on a lift to do this job properly. Could I do it myself? Of course and many here have done so, some more successfully and easier than others. I chose to turn this part of it over to the pros.
12. Emission issues: If you are in a state that has mandatory emission testing, you will not pass with a catless exhaust. I am not in such a state.
After reading and studying a lot, gathering as much information as possible from this and other forums and knowing that this is the fourth turbocharged car I have owned and modded over several decades (I went with bigger catless dp/rp's on all the others, all European sport sedans), I made the decision to go with this mod.
Result: The combo of a good intake and a catless dp/rp is a great power producing mod in states where you can go this route. Expect a huge gain in performance throughout the entire power band.
Attached is a typical cool weather accelerometer run. Compare these numbers to stock and draw your own conclusions. You can also use a simple stop watch and know that 60-100 times shrink down to the 6.3-6.4 second range under cool conditions compared to stock values of about 8.0-8.1. seconds.
