Looks tuff man!
R32 GTR's are already quite responsive.
Up the boost to just under 1 Bar (14.7 Psi) but depending on your twin turbo's condition, full turbo back exhaust as you said, upgrade your panel filter in your air box or go twin pods.
Upgrade suspension, strut brace, top quality brake pads and a nice heavy duty clutch are a must if you are planning to put some power into it.
I see you got the R33 GTR wheels and I believe they are 17" already, a good diametre but how wide are they? If you can fit wider wheels and keep it at 17" or go 18", that will help with handling.
Depending on how many kW's you want, I think it will be a while before you max out your standard 440cc fuel injectors...
APexi Power FC will come in handy with your boost tuning.