My main hunting rifles are a 20" 6.5 CM and an 18.5" .30-06, both chopped and threaded for suppressed use.
If the 6.5 were a few inches shorter, it certainly wouldn't have cost me any animals. I have a self imposed 400yd limit though, and the furthest I have taken animals was at 320 yds with last year's cow, and 375 yds with this year's cow:
I'm often hunting pretty open areas, but every other animal I've shot has been at less than 300 yds. At those ranges, maximizing muzzle velocity and even BC isn't terribly critical.
I would suggest being picky about what 6.5mm bullets you use on elk though. I know people that have killed elk with 6.5mm Berger's, ELDxs, etc. I'd use those on deer and antelope, but personally I want something of heavier construction for larger game. I've used the 127gr LRX and 140gr Accubonds on the two elk I've taken with my 6.5. The Accubond is probably my favorite all around hunting bullet, lots of damage and good penetration with high SD versions. The higher SDs regular 6.5mm bullets boast tend to help with penetration, I haven't caught any 6.5mm bullets, but I did catch two 180gr .308 Accubonds last year (to be fair, after they went through lots of meat and bone).
I'd probably go with an 18" barrel personally, but I doubt going 16" instead would cost you anything you'd really miss. Bare muzzle, 16", 18" and 20" are all going to be really loud in full power rifle cartridges, I don't worry much about the difference, but suppressors are kosher everywhere I'm going to hunt.