What is involved in getting the 25 mm Manifold Insulators to fit within the engine bay versus the 6.35mm and 12mm units? Also will the 25mm unit have stronger side affects up high versus the other two?

Assuming you have an LFX/LF1/LFW SIDI V6 you will need to modify/trim the firewall shroud which covers the plenum for the wiper arm assembly. The 25mm item produces a 10% improvement in bottom-end power and torque as opposed to a 5% and 8% increase for the 6.35mm and 12mm items respectively. There are no unwanted side effects to either of the products.