Early ARM11-based phones don't have L2 cache (iPhone/3G, G1, etc). The newer ones (Palm Pixi) do have some L2 - this can help significantly. The much better microarchitecture of the Cortex paired with L2 means phones like the 3GS/Pre/Droid/etc are significantly faster than ARM11 systems.

There's still a lot of detail in implementation which means that comparing MHz between Cortex A8's to determine system performance is also kinda meaningless. Cache sizes? Memory bus speeds? OS implementation? How much time is the CPU spending doing housekeeping, talking to the baseband, etc? GPU performance and efficiency of APIs?

Even with the same chip and OS - the Linux-running OMAP 3430 that is reportedly in both the Pre and Droid - comparisons aren't easy. The Pre runs at 500MHz supposedly, and the Droid at 550MHz. A win for Droid then on browser speed? Not necessarily - its screen has 2.6x as many pixels as the Pre - 73MB/sec of RAM bandwidth just to refresh it vs 27MB/sec on the Pre. Not only is this more rendering, it's sucking bandwidth away from, well, everything in the system. Maybe they beefed up DDR bandwidth? Who knows....

btw, the 3GS appeared to be 20%+ faster than the Droid at loading and rendering a webpage in the video review I saw. Same ballpark as the 3GS admittedly - as it should be with a Cortex - and a mile away from currently available android phone performance, but not quite there yet