I know that the rb25 head can flow big numbers with tomei poncams, extensive head porting and tomei valve springs.
My mechanic has achieved 484rwkw using the above mods to the rb25 head; that's on C16 race fuel, 35psi boost, no NOS, and using an rb25det (not an rb30det). I myself have done the above and have made 325rwkw, using a GT30R (500hp) turbo, 18psi and on pump fuel (BP Ultimate).
Having said that, it would probably be cheaper to sell the rb25 head and buy the rb26 head. That way you will also have the benefits of solid lifters, be able to run bigger cams and multi TB's.
The major cost is in the porting, as a good head porting job can cost you in excess of $1600.