Thanks for the straight answer. Was hoping to hear that Salix' strategies for keeping up with Slackware are automated in a generalized way, which might be applicable to other CPUs.
Will go with FreeBSD instead, then, on several old PowerPC Macs which still want to feel useful They're pretty darn snappy under KDE on Slackintosh 12.1, but need to keep up with security updates.
I would rather recommend Debian with Xfce - probably Squeeze, though Wheezy might be better if you want working Webkit browsers. I'm currently running Squeeze on a Powerbook G4, and it works quite well (with a bit of tweaking), though the hard drive is in the process of dying on me. Main issue is X11's shoddy support for the Radeon 7500M, which causes windows to render slowly.
NB: with recent versions of Xorg, you probably want to turn AIGLX off on legacy Macs. On the G4, the framebuffer would get messed up with EXA acceleration unless I disabled AIGLX.
Fluxbox will work, but the Debian Squeeze version is a bit on the old and buggy side. I personally prefer Openbox, despite the obnoxious XML configuration files. PekWM and E16 are also good, E16 especially if you want something easy to configure.
As for tweaks. The other tweak I've done (aside from setting dirty_background_bytes and dirty_bytes to something low, as described in the other thread) is to disable smooth window resizes in favor of outline resizes. Fluxbox of course already does this for you, and better than Xfwm4 - it doesn't grab the screen, so things can update while a window is resized. E16 also does outline resizes by default, and I believe PekWM can be configured to do so.
The main thing I'd worry about on an old Mac - especially when using the unaccelerated framebuffer driver, which is necessary on some of old video cards - is GTK redraws being painfully slow. Unfortunately, I don't know of any way to speed up GTK redraws other than using the (rather ugly) default GTK engine.
It might possibly be better to use a standalone WM with KDE applications; that way you'd get the supposedly better rendering speed of Qt without the bloat of Plasma.