As much as you might be able to tweak a stock rom, you can't change the SYS if you never flash it. Some builds of Windows are just inherently faster. In certain cases, it makes a HUGE difference. If you actually benchmark / compare file read/write speeds, as well as taking in account the 'snappiness' of the ROM.
I second the recommendation to go try Juicy 5; you will never turn back
Plus, it saves you time if you don't have to apply the tweaks yourself.