Thinking on getting rid of my LMT 10.5" SBR and replace it for a Piston upper .

try it out b4 you get rid of the lmt if you can.  my 10.5 lmt di upper is prob my favorite shooter.  unless this is for a dedicated suppressed gun i say stay with the lmt.  i've never shot a 516 but i do have a 10" sig 556 and if it was one or the other the lmt would stay with me , but that's just me YMMV

Yep, I have one, a Gen II.  I use it on the normal setting without a suppressor.   On the suppressed setting it does not give enough gas to function unless a suppressor is mounted...in other words, it is working as designed.  I have not had a need to use it on the extra gas setting.  Turn the gas off and it functions manually.  I like it.  Other than when I was experimenting with the suppressed setting with no suppressor mounted, it has functioned perfectly.  Steel cased ammo also functioned perfectly.

excellent , this maybe ignorant sounding but do all piston bolts fit all piston guns or is it a proprietary system

The BOLTS are the same to the best of my knowledge (some makers may omit the bolt rings though)...it's the BOLT CARRIERS that are sometimes different (the carrier key part where the gas piston rod pushes against the bolt carrier) since the piston rods will be of differing lengths from the different makers.  For example, the carrier key of my Adams Arms bolt carrier needed about .15" removed before it would fit and work in my Ruger gas piston upper.  I put the AA carrier into the Ruger upper so I could use it full auto on my M16 lower (Ruger carrier was semi auto only; AA was full auto).  

well that fixes that . One thing I love about the AR and AK platforms , its alot of plug and play . I think i'll just stick to my LMT 10.5 then

I love my 10" 516, personally.

I wouldn't worry about carrier compatibility. Outside of running it FA there's no need worry about a new carrier. I've heard that LWRC carriers work though.

The bolt of the 516 is the exact same as the M400.

I'm really starting to like having multiple gas settings. Run "P1" for standard ammo. Run "P2" for underpowered stuff like frangible or cheap steel. Run suppressed if you are using a can. Helps regulate the system to make sure it's reliable with any ammo you can use with or without a can while not overgassing the system. Look how many threads you see in the tech forums of "why doesn't my gun run with Uncle-Teddy's brand-X steel garbage"?" or "Why is my gun malfunctioning with a can?"

1iviper, the 556 and 516 share absolutely nothing in common and is at best an apples to oranges comparison.

i understand the differences in the two , i should have been more clear.  personally i would choose the di gas system over a piston system in an ar unless it was a dedicated suppressed gun , of course YMMV.