Hi Steve,
I have used malt past the 'good by' date (as in 2 years past); and the only difference I've noticed is lower efficiency, meaning I make my recipes to give me 10 points higher OG than what I really want, and I am usually very close to what I want. Mine are all uncrushed, I crush on demand... so that is the only difference.. however, the beers turn out great. I would say to proceed with at least one batch and just calculate for some lower extraction efficiencies.
* 2 years past date because of purchasing in bulk and then having long assignments out of the country and not being able to brew for some time.