From the specifications the 600 doesn't seem to be very different from the 580 - usually for Canon this is the first time that the number doesn't relate directly to the power
Well, the number is directly related to the guide number - the 580EX II has a guide number of 58 meters, and the 600EX-RT has a guide number of 60 meters. So, it's a modest increase in guide number (consider that the predecessor to the 580EX was the 550EX, and the predecessor to the 430EX was the 420EX).
The real problem is that guide numbers are not really indicative of flash power for heads with zoom capability, since the zoom setting changes the effective guide number.