sorry, I'm not using any car BT with the phone, but I suspect that, aside from the display, the hardware differences between the 640 and the 640XL are minor and I'd be pessimistic that it will work much better than the 640.
Thanks, because that definitely wasn't my experience with the 640. It works great with my 925, so I was disappointed when I tried the 640. It works but it doesn't sound good on either end of the call.