Has the 0.63 ever been a restriction under 280rwkw?
I would want as much twist on a GTX compressor as early as possible. Sure we know we can tune an RB and make them go ok with appropriate fueling and more timing down low but surely the point of going for a highflow sized turbo is to get some action happening early.
Tacoma Nissan on ebay is relatively well priced for a full OEM engine gasket set. Will include valve stem seals, plenum gaskets, cams cover seals, new exhaust manifold. While the head is off it might be worthwhile fully servicing the head. About 10 days from order to delivery.
Large framed turbo and good street response is what everyone wants but physics gets in the way.
If it isn't in that thread then it's probably not going to happen the way you want it.
Is it a silly question to ask why you need that power specific power level?
I spent a few seconds doing a search for you. I hope you find the answers you need here.
http://www.skylinesaustralia.com/forums/topic/251820-got-over-400kw-atw/page-153?hl=+400rwkw
I might be a crazy party pooper but what has ported heads, aftermarket cams, large turbos and RB26 got to do with response?
Surely, stock cams, -9's, dialed cam gears and E85 is about all there is to it?