STD turbo,s have a limit by the size of the wastegate holes as a std turbo, your get boost creeps of your exhaust system if you exhaust flows to much air as the wastegate holes will not flow enought air to control the boost , you can either restrict the flow with a race cat or a restrictor plate or run a STD downpipe
With all the right mods , exhaust sorted , mappable ecu, boost controller , STD injectors and air flow meters running around 1.1 bar boost your see around 410ish BHP flywheel or 340/350 bhp hub.
But with other mods like cams , intercoolers , injectors , Nismo AFM,s, larger turbo,s you can easily get 500bhp without getting into the motor.