Steve, the K6-2 is actually *slower*, clock-for-clock than the K6-II+, and it has only L1 cache, not L2 cache.
The K6-II+ actually has the same die as the K6-III+, but only 128K of the full 256k L2 cache has been tested working (the choice of which 128k to use is probably fixed by bond-out pads, post- wafer testing).
In short, in order of decreasing performance-per-clock:
K6-III+, K6-III, K6-II+, K6-2.
In order of decreasing suitability to use in an I-Opener, taking into account power consumption as well clock-multiplier issues and the availibility of k6clk.exe:
K6-III+, K6-II+, K6-III, K6-2.
Under almost any criteria (except power consumption, where the K6-III is worse), the K6-2 would be dead last on a per clock basis.