I upgraded my dad's iMac and also my iMac from Tiger to Leopard without reformatting and had no problems and didn't detect any loss of system speed after upgrade.
Haven't done a clean install of Leopard on either machine, so can't on how speed compares with my in situ upgrade.
I would just wait several weeks after Snow Leopard has been released and see if others had any problems with your wanted upgrade path (obviously should back up any important files and also consider disk / partition with Super Duper, Carbon Copy Cloner, or even Time Machine onto hard drive that you can completely disconnect from current Apple rig while update is attempted).