Thread is
http://forums.smallnetbuilder.com/showthread.php?t=17836
I have seen no indication of manufacturers trying to market higher power in the low 5 GHz band as a differentiating feature. NETGEAR's R7500 and R8000 and ASUS' RT-AC87U/R support the higher power.
You won't see this in older routers for the reasons I gave. The FCC can't be "sweet talked" either products meet spec or they don't.
As jlake points out, manfs have no incentive to go back and re-engineer old products. And sure as hell not for the slim margins they get. The history in wireless is new things WPA, WPS, Wi-Fi Direct (still waiting) get baked into new stuff and very rarely back-engineered into products in the field.