300rwkw is a walk in the park for a "good" R34 gtt engine we have around 4 all making 290- 330rwkw unopened all done for a lot less than 10-15k we even have a few std RB25's making 320-330 rwkw's using full JAP gear (ie Greddy T67 full external gate turbo kits, jap injectors, PFC, Plenums, pumps and titanium exhausts for less than 15k)
It all comes down to using quality parts and management.
Having driven 100's of skylines with nearly every concievable combo i reckon 280-290rwkw is about the sweet spot for a daily with DECENT rubber and rims. this equats to about 350-360kw if comparing to commo's and falcons.