I've only done decoctions when I missed my mash temperature, so when are you pulling the decoctions to add them back to the main mash for the full rest times? Or do you ignore the time it takes to get the decoctions up to temp?
Check Kai's discussion of decoction mashing here http://braukaiser.com/wiki/index.php?title=Decoction_Mashing and watch the videos, too. I've played around with a few different decoction schedules through the years, and use different schedules for different styles. I've always seen a little bump in efficiency except when using only a single decoction to reach mashout temp.