Depending on how much power you want...
Nistune can easily achieve over 300RW kW's for a GTR.
Considering your engine is stock, Nistune is enough.
If you want to push it further, go for an A'PEXi Power FC.
I think if you got a V-Spec II Nur, it will be painted JWO - Millennium Jade.
For a M-Spec Nur, it will be painted EY0 - Silica Breath.
Millennium Jade is specific for the V-Spec II Nurs
Silica Breath is specific to the M-Spec Nur.