Well I have a 5550 and a 5450 right here, side by side, and I tell you the screens are different. When on 5550 has better contrast. When off 5450 is grey, 5550 is black.
What your talking about is not unlocking the system bus chip. You are talking about overclocking. Yes there are many apps available - pockethackmaster is one (
http://www.pockethackmaster.com/). It allows you to overclock the 5450's pxa250 to 472Mhz and the system bus to 107(I think). Better performance at the expense of battery life. An xscale is designed to change speeds depending on usage, for maximum battery - all this app does is put the chip in turbo mode all the time. But the 5550 has the NEW 255 chip with the whopping 200MHz system bus (Intel were some what embarressed about the 250, everything runs slower that on the ARMs in the old pocket PCs). In fact in the case of Dell axims, the newly manufactured ones contain the new chip, but theres no way of knowing which yours has until you buy it. Obviously they dont want to be left with a pile of inferior ones. The new 5450 ROM upgrade adds support for "processor upgrade", however an upgrade is not so straighforward as swapping CPUs, other things have to be done.
So there you have it - different screen and different processor.