When your at 350Bhp other parts are on there limits so best to go for the full monty..

GT28RS,
Miltek or PD full system & decat,
PD manifold,
Dreamscience remap or Autronics,
Uprated fuel pump & injectors,
WRC Rods & Pistons,
Headwork & Custom made cams,
Nology or Denso plugs (colder grade)
Blitz or Apexi Boost Controller,
Frontmount Intercooler,
K&N induction kit,
or RAMAIR induction kit (if you go autronics as the old MAF is removed)

Should be good for 380-400Bhp

Depends on what your budget is too