Unfortunately, the k6 series of chips stink pretty bad at RC5.  Any added output is welcome, of course, but a k6-3 will have a little more than half the keyrate of an similarly clocked p2 or p3 chip.  In OGR (and Gamma Flux, too, I believe), however, the k6 series of chips kicks some serious butt.  They are every bit as fast as p2s and p3s.