Wait for a SAU or club "dyno day"
Just saw this thread for the first time VERY impressive and a nice selection of mods.
I too have a HKS GTRS kit; but running Tomei Poncams, head porting, std plenum and std bottom end. My a/f ratios were kept at approx. 11.5:1, to keep the engine "safe". Running 18psi i made 268.2rwkw, but your response is WAY more impressive. Could be due to the plenum, cams, fmic (i was using a truck core), but most probably to the fresh rebuild.
The engine has since blown, while i was at a race track (cyl #3 dropped compression), and i have decided to build an rb2530, which means upgrading the turbo, again. I guess i will never know what the GTRS's true potential is, but yours demonstrates it clearly. Well done mate, an excellent build and it would feel awesome to drive.
Below is a comparison between our two graphs, you will be happy:
For my new engine i have bought a new GReddy plenum, XTR 100mm fmic, Garrett GT30R turbo 500hp (smallest recommended turbo), ATI balancer and a whole lot of other parts. Unfortunately it takes my mechanic a lot longer than 3 weeks to build an engine, but we'll get there.
All the best and enjoy