As Matt said, no gain in adding an extra throttle plate, your current one is no restriction.
I doubt you will gain anything, infact I would say you will go backwards from where you are now.
Also why bigger injectors?
the NA injectors are 270cc, so good for around 270hp, which is a very serious RB25DE and you wont get anywhere near that with just an exhaust.
to get there think 13-14:1 comp, huge cams and hours upon hours of headwork and race fuel.