I think you and any one else who has one of these scrap metal spacers suffers from the placebo affect. Even the "best" claims from these things are about 5-10HP, something you'll hardly notice unless you only started with less that 100 to start with. Most popular engines people buy these spacers for have HP in the mid 200's to low 300's range where such a small increase will be negligible.
I watched my friend drop $180 on a spacer last year, I tried to convince him not too. But it was no good $275 later he had a spacer and 3 royal purple oil filters. It would have been better to burn the money or just hand it to me and help pay for my winch. OH WELL
While we're at it you lets add cold air intake kits to the pile as well. Only thing they are good for is parts for snorkel kits.