depends on who you ask as to what's needed with Stage 2+
on the Revo website it says required equipment on K03 cars is CAI, TBE, HPFP and intercooler
but the general rule of thumb is CAI, sports cat DP (or decat), HPFP. intercooler is considered a "recommended" mod, but not essential.
in terms of your exhaust, air flow is limited by the most restrictive part of your system. in this case, it'll be the OEM centre. BUT most of the restrictions are in the DP, so you should be fine. Nick at R-Tech even said to me that DPs are a waste of time, and that cutting out the OEM cat and replacing it with a generic 100 cell sports cat is enough.
tbh if you're wanting to push your car to the absolute limit, an intercooler and full TBE would be recommended. but if you're not bothered, you should be fine.