ooh I have been thinking about Leopard on my Dual 500 G4 - what's noticeable in the 'slower' department.
Tiger is pretty snappy to be honest, but there is a nagging feeling in my head that there is some sw I will use that needs 10.5+ (can't think what it is right now - blonde day)
The OP's idea of "Marginally Slower" may seem like an eternity in regards to number crunching and launching apps to other people. Those specs in both the OP and Costabunny's Macs are far lower than the recommended system requirements for Leopard. I'm sure the OP had to go through extra steps to get Leopard on his machine. It won't automatically install once it detects the system specs are below the requirements.
For the most acceptable performance it's best to stay with Tiger since it works just fine on Macs older than Costabunny's Mac.
-A Mac computer with an Intel, PowerPC G5, or Power PC G4 (867 MHz or faster) processor
-512 MB memory or more.
I only have 700MHz but I used this program to "tweak" the install:
Yes its not what you would call " snappy" but there's hardly any difference to my previous Tiger install. Did a little work with system clean up to get rid of some " junk" files etc.
Probably on a 500 MHz it would not work well. Anyhow I find Leopard great!