Does the latest Tetsu kernel release break anything with the latest Cacko?
No, I think it's included in the Cacko release (it is in the latest C3000 beta version by Iamasmith anyway).
What benefits are there other than the over/underclocking capability?
The top of Tetsu's page, although not quite easy to decipher, gives plenty of hints about what's been fixed/improved in Tetsu's kernel, for instance on the C3100:
Patch 01: under/overclocking
Patch 02: voltage adjustment (useful for overclocking, I think)
Patch 03: patch for SIGSTOP problem
Patch 04: UNICON patch?
Patch 05: patch to allow variable pressure on the digitizer (useful for drawing applications)
... et caetera.
So it's not just for overclockers, it's more or less the 'state-of-the-art', optimized kernel on Zaurus platforms. Of course, since the patches are freely availables, you can also build your own kernel with every option you like!
Does pdaXrom or Cacko offer any over/underclocking at all whatsoever right now?
Can't say for pdaXrom, as it was not yet mature enough to be usable on my C3000 last time I tried to have a look, but the Cacko ROM has an applet to handle under/overclocking, as far as I can remember, though I don't think it was included in the stock ROM. A forum search should give you more details, there's been a thread on this in the past, I think the overclocking part was included in the battery/memory applet.
Check those links:
https://www.oesf.org/forums/index.php?showtopic=11753https://www.oesf.org/forums/index.php?showtopic=10814And most importantly this one:
https://www.oesf.org/forums/index.php?showtopic=12413