need help with WZR-RS-G54

I'm trying to set up the Buffalo Tech WZR-RS-G54 wireless router with
Buffalo Tech WLI-CB-G54S client card and run in 125 High Speed Mode
(i.e. Turbo mode).

The problem is: While the two can connect in 125 mbps, I'm getting less
than 6.5 Mbps throughput. A file that takes 9 minutes to copy in
regular 54G mode is now taking 26 minutes to copy in Turbo mode!

Does anyone know if this Turbo mode really works?

TIA.

/c