According to http://www.smallnetbuilder.com/Revie...-F5D7130-5.php my simple measurement is OK.
On Frame Bursting the following text (from link above) make it more clear:
Turbo mode
Next up was checking Condition 1 performance using the Belkin card with Turbo mode enabled. Since "Turbo" / Xpress / frame-bursting is aimed at improving mixed WLAN performance, I didn't expect to see any improvement with a single 11g client and that's what I found. I did run some tests with one 11b and one 11g client, but since Broadcom disses Chariot-based testing for evaluating Xpress' performance and also stresses that you need multiple 11g and 11b clients to really see Xpress' advantage, I decided to put that testing aside until I discuss test techniques with Broadcom.