Software power measurements aren't terribly accurate. There's no way a 580 running 120MHz faster with double the VRAM is using less than a 570. There may be some 570s with hefty factory overclocks that could draw more than a non factory OC 580, but the vast majority of the time a 570 will draw less.
No, undervolt will not reduce clock speed. If you lower the voltage too much it will just become unstable. In fact, if your card isn't maintaining 1244 MHz consistently that means it's probably hitting the power limit and throttling a bit. If that's the case, undervolting will actually improve performance by letting it maintain 1244 MHz consistently.Under volt will reduce GPU clock mine on +0 power and it's barly can get to 1244 hz
About the fans I will have enough power supply for add 2 fans with 450w PSU?