The i5s, such as the 2500K DO NOT HAVE a single core, THEY ARE QUAD CORES TOO. apart from some 1st gen i5s. If you look at the 2500K non-synthetic benchmarks, you an see it beats most i7 first gens and is on par with many of the first gen i7 extreme editions.
(anandtech is down but I would have Posted a benchmark here.)
i also wanted to post some of Anand's benches.