What's the difference between your 12mm plenum spacer and 25mm plenum spacer? And what is the difference between the 12mm and 25mm manifold insulators as well?

The 25mm plenum spacer is designed for blown applications, for N/A applications we find the 25mm plenum spacer reduces throttle response to much and therefore recommend the 12mm spacer instead.  It's the opposite for the manifold insulators/spacers.  There we recommend the 25mm manifold spacer for maximum gains in torque and the 12mm manifold spacer for blown applications.