Just my opinion, but if you gained 2 tenths with nothing more than adding a Super Sucker, your original carb tune-up was off, probably rich on the jetting and/or the accellerator pump circuit. The many weather variables can also make quite a bit of difference from one day to the next - temperature, humidity, barometer, breezes, etc.
I haven't done back-to-back testing to see how much it helped in my particular combination, but I am a believer in spacers and use a 1" version by Canton. I switched from a 4-hole to the tapoered 4-hole, so the difference would have been less than going from no spacer at all. But any such changes affect the carb and can require retuning to "re-optimize" it - if it was just right to begin with.
Aliminum is more durable than plastic, but transfers heat a little more. Plastic is a better insulator but can warp from too much heat. The Canton version I use looks like machined plywood or particle board. Forgot if they call it "phenolic" or something else.
