The 2500k does not have 1/3 more power nor is it obsolete in any way.
Sandy bridge is clock for clock better than the older nehelams by 10-15%. Although it is slated to be replaced by LGA 2011 next year by SB-E, the fact is that it's still a top notch CPU and if you find it for a good price then by all means get it.
But yes the 2500k is better by 10-15% clock for clock but neither is slouches.