NDD,
I don't know how much you make it over to SOCNET anymore, but there is a running thread over there on this.
IMHO,
Forget HK and go with Barrett's 6.8mm SPC proposal. Save a whole lot of money by DXing the XM-8/26 and use the proposed "drop in" on the current lower frames. The only usable thing I can see HK bringing to the table is it's gas cylender tech, mesh that with Barrett's system and GTG.
BTW,
The picture above is HK's "drop in" with it's improvements untill the XM-8 is O-K'd. Why not try taking two birds with one 5.56mm stone?