You can get around the CPU speed issue to install Leopard easily enough, however it's probably not suited for the stock 450/500Mhz G4 - with a 1Ghz+ CPU upgrade however it runs well. There is a seller in China selling cheap Cube CPU upgrades at the moment.

The issue you may have heard of is that a PC flashed 6200 in a 10.5 Mac takes an age to start up - the workaround is to remove a particular .kext file to cease this lag. However, there isn't any issue with 10.5 on a Cube, even a stock one.

JB