My new 5600 has the PXA255, I noticed a difference in speed when I went back to the sharp 1.32 ROM after playing with Zynergy (problems with the graphical ipkg tool, not much fun if you cannot install new software without jumping through hoops) and OZ (problems communicating with my PC over USB, want to use it in work and have no wifi there)
The sharp ROM was very noticably slower than the original ROM that came with the 5600 so I am going to give the pre-emptive kernel a go, I am resisting the urge to co for the special overclocked kernel as my Z is still less than a week old !
So the quest goes on for a ROM for 5600 !