If you are referring to their BCM (their own brand) M4 barreled uppers, you'll be getting a true mil spec upper in every respect- F marked FSB, 4150, 1/7, fully chrome lined, gov profile, HPT and MPI barrel, M4 feed ramps, and LASER etched T-marked picatinny rail. If you get the CMT MPI bolt to go with it, it'll be the best complete upper money can buy (well, at least, per the spec on paper anyway).
The best part- it's a great company to deal with.
ETA: When I purchased the BCM 20" upper from Grant of G&R Tactical, CMT upper receiver was used. If that is still the case, a Stag/CMT lower should match nicely (if the deer head trade mark doesn't bother you). Don't know what you meant by matching the lower to the upper. If you were talking about matching the finishing color, who cares, man? It's a firearm, a tool; not a purse.
But hey, that's just me. To each his own.