Nistune does not have a "limit" as such. It can be used to tune to what ever your engine can make (200kw or 1200kw) but if you are flush a more expensive ecu such as Link or Motec will give you a few more features. Since you already have the Nistune just go with that until you have found a reason to replace it. I would get a GTR box first with the power you are looking to make. I presume you have already upgraded afm, fuel system, brakes and suspension.