Most AR15 piston rifles on the market are retro fits. They use the AR180 piston system that was developed in the 60s and retro fit it to the same round AR15 carrier, some with intragal keys some without.
The op rod with a return spring directly over the hottest part of the barrel is not a good idea, Pat Rogers says some break at 1500rounds and some break at 4000, it will break just never know when.
I have shot and repaired many AR15s with piston systems and I now own a SCAR, there is no way I would pay $2000 for a AR15 piston if the SCAR is available for $2500. The mil has gone to an aluminum lower on the SCAR which IMO was a good idea, the plastic stock seems a little cheap but I'll still take it over a retro fit every day. There are many small things inside the Scar to show it has been well thought out.