That would seem to suggest that the wireless is tied in to the Ethernet rather than separate. I've only used my Plus and Pro on wired connections but it would be interesting to see if they share the same speed limitation on wireless. If my theory is right they should not, otherwise it means the limitation must be in Tivo's wireless implementation.
Another thought, do we know if Tivo has multiple antennas and can hit 300 Mbps or only 150Mbps on wireless? That could be another reason for the transfer speed limitation.
Roamio Plus (3TB)
THD 2TB x 2
Humax S2 300GB (retired)