I don't want to start a "religious war" here, but I honestly don't see the point of SL_PPC.
It is in fact straightforward and does not need to rely on poorly measurable stuff like “speed”. Anything which uses libdispatch cannot build on Leopard, but can build and work on Snow Leopard (including PowerPC). More or less that means every relatively modern app using ObjC and a decent number of C/C++ apps.
The only reason, for example, why OneTBB still works (I hope) on Leopard is because I have fixed it, adding a fallback for dispatchless systems
For the same reason WoflSSL will probably still work on your “Sorbet” – I changed the logic of picking backends to the effect that old systems use POSIX one, FWIW (may not work reliably, have known issues).
I stopped doing that ever since. So unless you fix it for yourself, chances are that everything will be broken.
10.5 has inferior CoreAudio framework, so most of multimedia apps won’t build, unless you add a fallback.
Some apps using blocks can be built on 10.6 ppc, despite broken clang, since gcc-4.2 in 10.6 has basic support for blocks. So you get Cocoa GUI in Qt4 etc.
In my experience, it won't run SL apps, which are all Intel
It probably makes sense not to try running apps with foreign architecture. I don’t know what is “SL apps” though. Old Apple software? It is largely Intel-only, but not all of it. So you get some newer apps. However that’s not the point of course; why use archaic software when newer and better alternatives are available. For open-source “SL apps” is a meaningless designation – hardly anyone developed specifically for 10.6.