Would the 25mm manifoldinsulator fit under the strut brace I have? And how will having pacemakerextractors and high flow cats affect these items?

It should, depending on the strut brace, however I'd recommend that you measure the current clearance without the spacer fitted to see how much clearance you have. The pacemakers extractors and hi flow cats are both good supporting mods that will work well with the 25mm insulator. The 25mm manifold insulator will improve torque through out the RPM range and the extractors and high flow cats will improve the engine performance in the mid to high rpm range.