Ok, still no real luck in getting a better reception from the CUL-stick. I have exposed some of the radio parameters of the receiver circuit in the Protocol Analyzer settings, so I can try different settings easier, but I have not managed to get very much better signal.
The only theory I have left is that the USB sending routines spend too much time in interrupt disabled mode so it disturbs the interrupt driven sampling. So the last thing I will try is to bypass the higher level sending functions and program closer to the metal of the CPU to send the data. If does not work, I think I will give up the CUL for reception.