I had 1" on my (Original) cobra intake to clear my black Ford Tall covers. But it ground the corner into the hood (bad mount not the spacer)
Now without a spacer, My Throttle body grinds into the hood with the TPS screw. Stupid Professional Products TB! If your mounts are bad, you'll know for sure when you lose clearance up top. I would recommend the 3/8" for normal use, and only the 1" if you have a cowled hood.
Well, My car ran smoother when I had it on. Did I notice a difference? Hard for me to say, because I did a complete Head/Intake/Mass Air conversion on the car.