Jesus dude some of those prices... You took a fire hydrant up the ring big time
Anyway - Still haven't seen mention of what cooler you are actually running?
You mentioned stock RB26 plenum, but you mention an after market one?
What set-up is currently driving the splitfire coils?
I also doubt 18psi was not making any more power, I mean its a 1000hp turbo. A base point would be 25psi and well into the 400rwkw region.
If any more than 18psi was not showing anything, clear as day there is a problem. Could be something as simple as cam gears not set correctly initially, or something much bigger. Side point - cam gears incorrect could also explain the very low comp test combined with your head mods.
Wouldn't get too worried at this stage, frustrated fair enough, but its just a matter of working through things bit by bit.