-10s are the same as GT-RS's - 450rwkw+
If 2530's will make 400-420rwkw, and the GT-RS are bigger again... Then pretty easy to set expectation when there are a few RS results making some BIG numbers in the dyno threads.
4" CAT will be a restriction on a turbo setup that large. Have a look @ any CAT, it can't NOT be a restriction and hence if you wish to run one you'll need a larger one for good performance.
Realistically though, for best performance with those turbos, only option is a 4" straight pipe - no CAT.
Remember RS's are internally gated so you need a lot of flow.
Realistically RS's are not really a great turbo anyway (hoping to see Piggaz comparo soon of -10s vs -5s).. That will hold a very interesting story. Same as Gav from WA, he picked up something like 1200rpm extra response doing the same step back in turbo size... Such a boost in the mid range is just insane for a street car