Cara if your after 300 at the moment then you will something like a GT3071 + cams +porting + lots of boost and you may just still fall under 300kw but it will be more responsive that the other option which is a GT3076 which will make 300 with cams and has the potential to make more.
As for the RB25/30 build, if you are after a big power engine then do yourself a favour an go for a 26 head straight away. 26's have all the good stuff straight away like solid lifters and cams to suit as well as not having the VTC gear which you can't run with big cams. Although the NEO's have the solid setup they have a problem with an exposed oil gallery once you go over 9.25mm of lift and therefore your potential power is limited. The other good thing about the 26 heads is the availability of aftermarket parts like a wide selection of cams, valves, springs etc etc.