The relay capability is only good when the satellites and rovers can see one another. The satellites are in near polar orbits with a period approximately 2 hours long. There will be a lot of time when the satellites can't see the rovers amd the rovers may be contending with one another for relay capacity (being optimistic that both rovers land safely). In addition, there's still the need to relay the data back to Earth. The relay capability is only as fast as the slowest link. In this case, it'd be the uplink from the satellites back to Earth. There are still visibility issues between the satellites and the DSN antennas coupled with the need for the satellites to transmit their own mission data back to Earth. The satellites can only see the Earth for a portion of each orbit.

Don't get me wrong, the relay capability is a good thing. It keeps us from having to put expensive and heavy high gain antennas and pointing systems on the rovers. Still, the ultimate data rate will be limited by the uplink from the rovers to the relay and the uplink rate from the satellites back to Earth. My memory may be faulty, but it seems the data rate is still in the low kilobits per second for that system.