I don't beleive the octane rateing would increase MPG at all unless your timing was too far advanced for your 87 octane or if you had so much compression it was pinging on 87.

What I am saying is that you can increase the compression at least 1 full point if you are willing to run premium. Increased compression is pretty well documented to increase MPG a very noticable amount. It is also a proven way to increase HP and TQ. So in short you run higher compression and higher octane together to get better MPG, yes it costs more but the increased MPG offsets most if not all of it.