I'm not sure on that. Look at what sidehack was able to do with a chip. I mean it's able to do 8GH easily with no issues.
While the Compac will run passive at 8Gh/s, I wouldn't really recommend it - never mind if putting it into an enclosed space with limited surface area for cooling (which the R1 largely appears to be, ventilation holes notwithstanding).
I was surprised how hot the heat sink got when running a compac at only 125 freq, I put a fan on the one I have at that frequency right away after I touched it.
Still think they should make a U4 with the new chips.. I need another small miner.
Given the track record with the U3, I would say thanks but no thanks to a U4. Instead, wait for gekko science to make their pod (
link). It will be what you are hoping for from a "U4", except the gekko version will work and you'll be supporting "the good guys" instead of a major international corporation.
I just bought their USb miner.