No, the reason it is rated at 2.8 and not 2.53 is because it was precision engineered and manufactured to be 2.8 and not 2.53.
Processor design and manufacturing isn't about making a bunch of processors then determining how to rate their speed based on tests, it is a very tightly controlled and predictable process. That's why they pay their engineers and physicists the big bucks.
I am not a computer engineer, so I may be wrong but I based on information I have come across, different speeds on a line of processors are achieved by changing the number of transistors and such that they squeeze onto that chip. Which is why you pay more for faster speed: it is more expensive to manufacture.
The reason they will not put a 2.8 in the 13" is as chrmjenkins said, to differentiate product lines. Eventually I am sure there will be a bump to 2.8 or something speed-wise equivalent based on what technology is available at the time.
Unless you have a very specific reason for wanting the higher clock speed (such as daily rendering of HD video or some sort of engineering simulation programs, etc.), it probably will not make much of a difference to you.
Safari, iWork, etc. which you will be using for your university course will load just as quick (especially if you get the SSD) on any of the Macbook Pros.