Is there much extra gain by putting on the 25mm manifold insulator over the 12mm manifold insulator I have on now on my VE Alloytec?

Generally speaking, you would be looking at improvements in torque by around 10% and 13% for the 12 and 25mm manifold insulators respectively. The feedback people have given us by upgrading from the 12 to 25mm manifold insulator, has been genuinely positive.