I'd say that the 18" would still be a good compromise.
The idea of a rifle gassed 18" is that it will behave better with a can than a full 20" unit. If that's a concern, 18" has that added advantage (in addition to 2" off OAL), and SPR's are generally ran supressed, whereas the SAM-R is typically just the open birdcage up front.
As for the idea of making a 16" Recce style run-gun setup, it does make sense and logically check out, but there aren't any production military versions of that, so that would be a concern. The 18" (especially a KISS oriented Mod0) would be comparable in weight, while retaining some of the longer range capability of the 20"
If I was in that position, looking for maximum range, my choice would still be a SAM-R impression, but with LaRue rail (or complete stealth upper), and a Leupold LR/T or Nightforce 3.5-15x as the optic.
That said - my rig is a CMMG 18" with TROY MRF-RX rail, UBR stock [right now wearing a cheap Millett scope]