ARM8 is not the same as Cortex-A8, which is essentially ARM12. While some A8s are partially super-scaler, notice they say "up to"; not all instructions can be simultainously dispatched. In fact, most can't. Most XScales have large caches and very fast interconnects (The OMAP range have a slow local bus and SPI). The new Ferocean/Sheeva XScale cores are meant to be much faster. My bet is that they're similar in performance.