Traffic analysis and inconsistent results on Nexus One
After some days of struggling I've come to the understanding that my Nexus One's NIC was corrupted/broken/something_bad_else. My tests were all messed up and with no logical connection.

I've tried with a S II device and results seem to be a little more consistent. By the way, even in this case, I can't achieve a worse throughput increasing the bit rate, actually it's still floating around 90% (from 88 to 95 randomly).

Probably it is dued to wi-fi instability and UDP packets instead of TCP. For my purpose having tried with a different device gave me a suitable answer.