Mine pretty much stays at 270Mb, but it's only going through 1 wall. Antennas parallel to the car and the dongle is on the top of the windshield behind the top factory tint.
All the settings are mainly default. 5GHz is disabled. Mixed (b/g/n), automatic rate, 20/40 MHz bandwidth. None of that helps improve data rate. Maybe a fixed channel since there are so many people using the frequency, but I think they channel hop anyway.