GTR's are much better, as there igniter module provides much stronger spark. ND4SPD was running stock coils and module when it made 500rwkw, so it's more than doable. Pity they didn't make the GTS-t's/GTT's as good:(
Bugger:( The Series II's always struggle to make more than about 330rwhp without running into spark issues, so we swap them for Series I coils and module or Splitfires.
My standard coils were working fine, but with higher horsepower applications then the SplitFires are the way to go:D Series II GTS-t coils are pretty crap though, as they have the modules inbuilt, and are much more prone to problems.