is there ANY benefit to a 90 minute mash using only 2 row.. crystal 20 and crystal 80? I am mashing right now and wondering why I am doing 90 minutes rather than 60???
You mash for 90min to increase wort fermentability. You may have full conversion after 60 min, but the additional 30min give the enzymes more time to break down the sugar chains so that the yeast can more easily digest them. Longer time at lower temps equals more fermentable wort. It's not necessarily an issue of effeciency.
EDIT: check out the mashing theory wiki. It will help you understand the process and give you even more control over attenuation etc.