I used to have problems saturating my broadband on 802.11n as I recall, even though WiFi to LAN was dramatically higher than my broadband connection. I never could understand why that was, I assumed some weird conflict with how it was handling the traffic.
Since moving to AC though I don't think I've had that problem. The fact different hardware is causing the same issue is puzzling, I could only think there is some very odd interference in the 5Ghz band that is killing the WiFi performance.
I do sometimes get an odd speed reduction across the road between WiFi to LAN and WiFi to Internet, but that is going to a 5Ghz AP, then over a 5Ghz PtP link, so there is a lot of scope for thing to go awry even when the channels aren't overlapping. I think maybe latency differences and packet loss over all those layers can cause strange issues.