Hi, RMerlin!
On your 27 beta I have same issue with link rate as on official beta .354...
Maximum link rate on 5Ghz is 108 mbps only and real upload speed is ~50 mbps.
But with the same settings on router and client on previous FW i got 300 mbps link rate and real upload speed ~75-80 mbps.
With the same settings on 2,4GHz band a get 300 mbps link rate on both FW (26b and 27 beta).
P.S. I`m alone on 5Ghz band with no neighbour wi-fi AP, except my N66 router
I cleared nvram, router was reseted to factory dafaults, channel was set manually to 36, 40, 44, 50 with no link rate and upload speed changes.
Settings wasn`t changed on client between upgrading FW on router.
Client is Broadcom 43224AGN 802.11a/b/g/n 2x2 Wi-Fi adapter. Driver version on client is 5.100.82.143
its on 27 beta
and its on 26b