I'm running the catless TurboXS dp/rp combo into the stock CBE. This is sometimes referred by TurboXS and its vendors as "Stealthback." I am very happy with this. I've put over 41,000 miles on the car and have had this exhaust set up for all but the first 10,000.
It is going to be very difficult to generalize an answer to your questions. I'll try, but would request that you read all the way through the several threads on this subject.
My view is that turbocharged engines DO NOT, I repeat DO NOT need backpressure to make more power. Any restriction on a turbocharged engine is potentially power robbing.
The problem is not with back pressure. It is with the potential for overboosting on the stock turbo if the exhaust gets so free flowing that the wastegate gets maxed out and cannot dump all the extra flow back into the downpipe. Some guys running aftermarket downpipes and race pipes have reported overboosting/boost creep climbing above their target boost levels. Those who have had this problem have put some restriction in the system to reduce the flow to control this. I've not had the problem, but acknowledge that others have. There's only one way to find out, because we've learned that each of these cars is a world unto itself and the same mods seem to produce slightly different results from car to car.
You can always add a high flow cat or put the stock secondary cat back on in place of your race pipe.
Another consideration is that you absolutely must monitor boost and probably should be measuring other engine metrics. A downpipe/racepipe, especially if catless, will have the desirable effect of raising your boost level about 2 psi. The increased flow and increased boost means more power. It can also mean exceeding safe boost if you do not monitor. IMHO, you want boost to be in the 18 psi range max to stay safe on this turbo, especially if you are on the stock tune. In cold weather you are likely to run into load cut at about 17.5 psi. Unless the load limits are raised (modestly with Hypertech tune, or more aggressively with AP tune, if you desire), you are likely to run into a wall and get abrupt loss of power when you cross that threshold.
Another consideration is the difficulty of installation. It's not terribly difficult, but it ain't easy. The particular location of our turbo makes gaining access to studs and nuts difficult. You must use a LOT of something like PB Blaster and give it plenty of time to loosen them up. Same thing with the oxygen sensors. That front one is broad band, is very expensive and is absolutely necessary for your engine to run properly. Take your time, go slow. Better yet, get a good aftermarket race car oriented performace exhaust shop to do it for you. I have a complete shop, lift and just about every hand and power tool I could want, but I still farmed it out because of the risk of stripping out threads.
You can run even a catless dp/rp on stock tune successfully. Don't let anyone tell you otherwise. I did it for about 30,000 miles or more and only recently went with the Hypertech tune. I was getting very quick transient spikes to 20-21 psi on stock tune. They would hit that peak when just coming up on boost and then immediately settle to 17-18 psi. I don't think that caused any harm. Others might say that even transient spikes to that level could be harmful. I just don't know. The HT tune did eliminate the spikes for me and I see a nice sustained 18-19 psi boost all the way to 5,500 rpm now with a nice taper to about 16 psi at 6,000.
With your FMIC you may be more prone to boost creep, overboost or boost spike than me, as I'm still running on the stock TMIC. My prediction, based on my experience is that with the combo of your good intake and a good dp/rp, you will see your 0-100 mph times drop by a full 1.5 to 2 seconds. It's a good mod. Not for everybody. Must keep an eye on boost levels all the time.
