Up to 1 gbps...there are "guaranteed speeds" for specific conditions, I think is 20mbps or something. Not sure with new 1gbps would that "guarantee" go up...
You see, their promise is "if both sides can handle 1 gbps, we will give you 1 gbps" but internet is not a perfect world, so that scenario is highly unlikely even if you have another MR user next door (then again, that would probably reach nearly theoretical 1gbps)...
Edit: Wait, won't reach 1gbps if you have MR user next door, since the 1gbps plan is 1000down 500 up, so would be getting like 400ish to near 500 I guess.
For the equipment that MR is most likely to be giving to end users and corporate users alike, it will most likely comes with a 1Gbps (1000Mbps) Ethernet Interface. This is the only main reason why no one will be able to reach the magical 1Gbps transfer speed over the internet or even on LAN.
Our equipment (NIC/Switch or Switch on the router) is only able to perform at a max of 1Gbps on the OSI L1 level only. Even top end enterprise gigabit switch will run into this 1Gbps wired speed limitation as well. Thus if the L1 transmission rate is only 1Gbps, if you include the L2 and L3 overheads, this figure quickly reduce to less than 1Gbps. Thus even in theoretical scenario, you cannot hit this speed even using leaner transfer protocols like FTP, TFTP.
The only way to get 1Gbps in usable bandwidth is that your physical interface must be more that 1Gbps. The next step after 1Gbps is 10Gbps and that means ONT and NIC must be 10Gbps. Equipment with 10Gbps option is not cheap nor is it a common place item for normal home users.
That said, with a 1Gbps connection and the right hardware (NIC and ONT and cabling), when conditions allows, 900+ mbps should be possible. Of course if your using SOHO grade equipments, you can forget about this figure as well. However, 700+ mbps should still be well within a reasonable expectation.
Review of 8 port Gigabit LAN-LAN Performance