I would like more down low punch as I have standard gear ratio. Is there much difference between the 12 and 25min manifold spacer in performance wise"

If you are running boost we recommend a 25mm plenum spacer and 12mm manifold spacer, as the thicker plenum spacer does offer even better air distribution between cylinders, which is more critical in boosted cars making lots of HP. Performance wise the 12mm offers 10% improvement compared to 13% for the 25mm spacer according Mace.