FYI I just got one today, after 20 minutes it gets to 2.06 Gh/s with clock=x0981, and it runs great with zero errors and zero rejects.
When I overclock to 0A18, which on the U1 is 2.2 Gh/s, after running for 20 minutes it only gets up to 2.12 Gh/s with 0 rejects and 5.41% error rate.
So, it seems like 2.0 Gh/s is the max you can expect with low error rate.
I haven't been able to find a user manual on the U2 yet however, and the clock parameters for overclocking may be totally different than what was used for the U1.
I have emailed Antmain about this but have not heard back yet.
Edited to add:
After 2 hours it's hashing at 2.063 Gh/s, with zero rejects and zero H/W errors.
Also, FYI my USB power is not an issue. I am getting my +5V from the molex connector of an ATX PSU at 32Amps.
if you had a u-1 and a u-2 to compare you could see if it has the same resistor controlling the voltage allowed to the chip.
Based on you tests so far I think it is the same.
which makes the older sticks better in one way the will fit on a tighter hub.
the newer sticks may not need fans.
So if you have a choice of sticks some may prefer the new for the silence offered. Some may prefer the old for the smaller size.