[PROBLEM] 300n-wifi has only 108mbps

[pix3l]

Hi everyone,

Software
XBian version: XBian 1.0 Beta 2
XBMC version: XBMC 12.2 (Frodo)
Overclock settings: none

Hardware
RPi model (model A/B 256mb/512mb): B 512MB
SD card size and make/type: 32GB Class 10 45MB/s
Network (wireless or LAN): WiFi
Connected devices (TV, USB, network storage, etc.): USB Storage

Logile/iwconfig ouput

Code:

wlan0     IEEE 802.11bgn  ESSID:"My WiFi"  Nickname:"<WIFI@REALTEK>"
          Mode:Managed  Frequency:2.472 GHz  Access Point: AB:05:54:52:5C:D8
          Bit Rate:300 Mb/s   Sensitivity:0/0
          Retry:off   RTS thr:off   Fragment thr:off
          Encryption key:****-****-****-****-****-****-****-****   Security mode:open
          Power Management:off
          Link Quality=100/100  Signal level=48/100  Noise level=0/100
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

Problem description:
I got a CLS WiFi abgn 300Mbit/s USB adapter which worked out of the box. Although iwconfig says "Bit Rate:300 Mb/s" the speed I get when I copy files to my USB-HDD is about 1.45MB / sec. My router tells me that the Raspberry Pi is connected with 108Mbit/s (which is plausible at a speed of 1.45MB/sec.) . And yes the router is wifi-n compatible. Do I have to activate the 300MBit somehow? Do I need to install a special driver?
Has anyone real 300Mbit WiFi working on Raspberry Pi?

Thanks for your help