I'm trying to decide on a muzzle device for my first 6.5G build. I have a 16" barrel coming that has 5/8x24 thread pitch so I'm looking at all the AR-10 devices..standard A2 may well be the way I go but does anybody have any compelling reasons for a different device for 6.5?

Whichever one fits your suppressor..

I'd probably go with a flash hider on a 16" or shorter barrel.

I have a Phantom flash hider on one 16” 6.5 Grendel and one 16” Faxon 6.5 Grendel with an A-2.   The 6.5 Grendel has such mild recoil I think the A-2 is fine.   I put an A-2 on my 18” 6.5 Grendel and my 20” 6.5 Grendel has a plain muzzle.   I really don’t care for muzzle brakes but it boils down to personal preference.

I run either Surefire warcomp or muzzle brake on my Grendel's and all other 5/8x24 thread rifles so I can swap suppressors around.

Current muzzle devices I have on my Grendels:

* LaRue Tranquillo (the most pleasant to shoot, like a Dead Air brake)
* ThunderBeast Arms Flash Hider/Brake Attach waiting for can to join it
* Precision Firearms NOX Flash Hider Nitrided
* A2
* A2
* Bare/no threads
* Bare/no threads

Others I have used specifically with 6.5 Grendel:

* PF LMD
* TBAC Brake
* Dead Air Keymount
* YHM QD suppressor mount
* AA slip over flash hider

For performance as far as maintaining sight picture goes, I like:

* TBAC brake
* LaRue Tranquillo (least obnoxious side signature)
* PF LMD

For just a flash hider, the PF NOX and TBAC units are really nice.

For economy, an A2 is hard to beat.

The Aero Precision 6.5 Gamma is superb
It's loud but looks fantastic and has great muzzle control

alas, no can but I do need one

as far as cost per efficacy goes....it is kinda hard to beat an A2 but there is always something nice you can get for a deal...hence my search

compelling device....won't get you any friends at the range but who cares about that?

I put a PWS Triad on my 12.5" but I haven't shot it yet.  Looks a lot like an A2 but cost quite a bit more

Agreed.  I have a Gamma on my 16" 6.5G and an Epsilon on my 11.5" 5.56.  Recoil is basically nothing, but my range buddies always ask me to warn them before I go hot so they can step back from the firing line.  I don't think it's THAT bad, but I've been shooting with brakes & comps for a while so I'm used to the concussion.  
Maybe I can convince them to buy the CAGE device for me...

Sadly, the CAGE isn't compatible with the 65 or 762 brakes.