I am unable to inject frames at any rate other than 1mbps.
I've tried with Andy's packetspammer [1] and with other pcap based
code (like 'aireplay-ng -9 -B wlan3'), but while the frames arrive at
the interface with the radiotap header set to other rates, they always
go into the air at 1mbps.
I have tested with Linux 3.13 and both ath9k and rt2800usb cards.
Cheers,
Nuno
[1] http://wireless.kernel.org/en/users/Documentation/packetspammer