I dont have a gt-r but i can offer comment on the HKS EVC5. I rate this controller highly, it does exaclty what it should do and has bugger all spike, even in different atmospheric conditions. It's steppor motor is 3 times the size of the apexi one, and the stepper, controler and the display unit each have a 64bit processor in them, pretty tech. stuff. The worst spike i ever had was 0.05bar, with the odd spike of 0.02bar (0.3psi) here and there, i've seen avcr's spike as much as 0.2bar, which is about the same as what you're unhappy about now.
EDIT: my evc5 also holds boost plus or minus 0.02bar.
-rb25