Not sure I’d call 940/35Mbps gigabit (better providers use that to refer to 1/1Gbps — Charter’s marketing wank doesn’t sit too well :/)

Comcast is at 1gbps/35mbps.

What's the overhead of ACKs? It feels to me that you probably couldn't saturate the connection without getting buffer bloated by ACK traffic, at which point can it really be called 'gigabit' if it's not theoretically attainable?

ACKs are TCP, not IP. They only claim 1gbps for IP traffic, not TCP.