usb_dev_bulk bits per second

Hi, I'm using an LM3S3748 eval kit (keil). I have modified the usb_dev_bulk/usb_bulk_example codes to do a looping IN transfer (ARM to PC), and am getting ~2 Mbit/second speeds.
I was hoping for a bit more, maybe 6 or 8 Mb/s.
Does the 2 Mb/s sound correct or am I doing something wrong? I have not done anything to optimize the transfers on the ARM side; it is still doing synchronous writes (via SendUSBPacket in the example code), and the PC side (w2k) is doing synchronous reads (via the WinUSB driver stack, which i have not used before either...).
Anyone have a speedier example using async i/o, double buffering or DMA or anything else on the ARM side?
Any thoughts, comments welcome.
Thanks,
Richard