What sort of throughput did you see with stock Google firmware before you installed OpenWrt?
You won't ever see 800 Mbps. For 2x2 AC wifi, 866 MHz is the maximum 'link' speed. That's not the same as transfer speed in real world. Usually it's about half that value.
Device page quotes Google Wifi uses IPQ4019 SoC, so performance will be similar to Linksys EA6350 v3 and EA8300 which use IPQ40xx SoC. I know my AC1200 EA6350 v3 can reach 500 Mbps using speedtest.net with Linksys firmware in 'router mode' and 2x2 wifi. But it struggles to reach 350-400 Mbps with OpenWrt in 'less demanding?' AP mode the last time I tested it a few years ago, due to lack of hardware acceleration in OpenWrt etc. It's even slower in 'router mode'.
in most use cases are as an extender so you have to 1/2 that possible 866Mhz
in best case anyway
and as an extender to extend it can't be that close anyway