There's some very long threads on several forums regarding spacer performance, or lack there of. Seems open work better on the big end and holed work better on the low end, and tapered seem to work better then standard open or holed because of the vortex created under the carb? Like Leoman said, without empirical data to back up the difference, it would be hard to tell on the street. Most agreed benefit is shielding the carb from heat! That being said, I run a set of 2" tapered aluminum spacers on my FE with dual quads on a lo-raise intake. I put these on to get more clearance for an aftermarket distributor. Can't say there was a real seat of the pants difference, but sure looks cool!