Generously 13k for that upgrade? I think i know what's the problem is Nark:D .
We drive our car everyday and for that we're so used to the power, handling....ect...that we sometimes get sick of it and want something NEW.
What we need is a get away ( from our belove skyline ) for a week or two or even a month, go over sea, go where ever and after all that time away from it, tell ya what it's feel awesome when you jumb back onto that seat, throttle feeling....ect...you'll feel like you car is ready to run 10sec ( well happen to me when i left to LA, seeing a jungle off Honda over there made me feel sick )
IF i were you Nark, i would ignore the rb25 idea, i mean it's great, BIG power but for everyday driving thing, 12sec on street tyre is enough and i'm sure RB20 can handle that easily.
You already have great exterior now spend 7k on the engine and coil-over stuff ....then get a premium sound add a TV for traffic entertainment hahahaha.