i just unrooted and locked the bootloader. im going to the verizon store tomorrow to attempt (and succeed) at exchanging devices. This is because i was in the same place (very very 4g strong area) as my friend, who also has the gnexus on the same 4g network (different roms though but shouldnt make much of a difference) with the same leaked new radios and we both did a speed test. he got 26000kbps while i got around 17000 kbps. i also lose 4g every once in a while and get thrown to 3g or no data. If they say that they will fix it in the next update, i will say this story and say it has faulty hardware.
we did many speed tests, both on 4g and 3g, and his beat me everrryyy time.
Let us know how that works out for you. It's the network.