Mil-spec in what way? There's dozens (maybe hundreds) of mil-specs covering various parts in a top end, but there's no mil-spec for a top end, espcailly a semi-auto top end.
That being said, our uppers do not meet every mil-spec out there for parts and finish. The main one being that we don't chrome line our barrels. Machining specs DO meet the mil-specs, however, which tend to be the most critical specs. Simply put, there's no one that makes a true mil-spec upper. I know what you mean though, and the answer's no.
As far as the barrel lengths, I believ the velocity loss between the two is anywhere from 150 to 300fps. There's usually not a lot of accuracy difference at ranges below 300m between the two.