Exactly! That 24 you were getting on your 480 was before strapping and tweaking. So, don't be sad with your 480 because apples to apples, it's very likely going to be a better performer (hash-rate to power ratio) than a 580 as those will use more power but only provide very marginal (if any) higher hash than a relative 480.  There are numerous comparisons hitting youtube where you can see for yourself. The 500 series are simply a re-branding of the 400 series so its not a surprise to most that there is no big gain. In fact, the 470s still seem to be the king of altcoing mining (except for ZEC)... especially with all of the driver issues plaguing miners trying to get 6x500 rigs up and running. Yes, some experts in power management may be able to get their 480's close to the power draw of a 470 but for the vast majority of miners, the current order of best ratios is:  470, 480, 570, 580. Personally, I would be very wary of going gung-ho on 500 series GPUs... especially that power-sucking 580.