RAM math makes it impossible to do a direct head-to-head comparison since the 3400c has 16mb on board and the Kanga has 32mb. That gives a max of 144mb for the 3400c and 160mb for the Kanga.
To be fair, my Kanga is maxed at 160mb and my(working) 3400c has 48mb. Even so, the Kanga feels more responsive to me on 8.6 than the 3400c does on 8.1. Apple claimed at the time it was "twice as fast" and I'm sure that was based on some sort of benchmark.
From my own experience, ~250mhz G3s seem to outperform 200mhz 604s in the same system(i.e. installing a G3 card in a 604 system). Given that difference, I'd expect a 250mhz G3 to be quite a bit faster than a 240mhz 603.
I can also pretty safely say that my 250mhz 6500(603e-based) is a real dog under OS 9.2.2 while even a 233mhz beige will handle 9.2.2 with ease. Of course, I've also found OS 9 to be somewhat RAM-hungry and the 6500 tops at 128mb.