What would Mace advice I should get next for my Holden VS Ecotec V6. So far it has 2.5in cat back exhaust, mace cold air intake, stage 2 shift kit with corvette servo and a tune. I was thinking a 12 or 25mm manifold spacer or throttle body. What would I g

In regards to gains, you would achieve greater gains down low (at lower engine speeds) with the manifold insulators, while the throttle body would mainly improve gains top end. In saying this about the throttle body though, it would also improve throttle responsiveness greater then what the insulators would achieve. This insulators wouldn't require a retune, however the throttle body would. In conclusion, selecting between product items would come down to your driving style (for example how high you would comfortably rev out your engine to on a consistent basis) and budget.