Hello,
I have 2 iPowers, one for my iUSB (9v) and one for my Ami Musik DS5 dac (5v).
They both make a high pitched noise, the 5v iPower being the worse (the 9v is ok, your really have to put your ear on it to hear it). I managed to "isolate" them so that I don't hear the sound anymore, with success. However I contacted ifi to know if keeping the noisy one was dangerous and if I should return it (I don't live in the UK), I am waiting for their reply.
As for the improvements in SQ, they were quite audible, both for the iUSB and the DS5 dac (I tried the iUSB first, then the DAC).
For my DAC, the iPower was better than my Sbooster linear PS, which cost me almost 3 times the price of the iPower ! The sound is even clearer, it's scary haha, I highly recommend it if you want to upgrade your DAC.