I guess there's more to it than "faster". How about lugging and torque?

 

ive rode a 300ex and i own a 250x, and their isnt much difference in the two. i belive i felt a little more zip with the 250x.

 

i have a 99 300ex thats slighty modded and my buddy has a 87 250X and i blow him away in the trails,but in open fields and running down roads,not to much difference,i think its what your wanting to do to it....

 

I raced both of these machines for nearly 6 years before the 400EX came along.

The 250X is a revver and the 300EX is a torker.... Same exact cylinder bores. Just different stroke lengths.

Top speeds on both machines in stock condition are VERY close if both machines are in good running shape. Any big differences there and one machine is faulty.

The 300EX only has ONE...I repeat...ONE,.. more pony than a 250X engine. However the 300EX is carrying almost 10 pounds more extra weight. So you basically have a draw in the power department.
MXers tend to like the 250X more so than the 300 and woods riders prefer the 300 due to the extra feel of power in the low end,...along with having electric start.
To the average recreational rider however, these differences are so minimal, one should really only take the electricstart factor into consideration in my opinion.