well i have a 1992 xr250r and my brother has the stock exhaust pipe for an rmz250. my question is- if i get this fitted will i notice and power difference? im thinking this because the stock rmz250 pipe would almost be like a sports pipe for my xr

There is every chance that you will notice an increase in happiness, sorry i mean performance.
In my experience a power pipe muffler is just a free[er] flowing assembly, sometimes they get made to a specific dimension to help with power output to suit a partic. bike, but generally if it flows easier and still controls the noise you can't lose.

Bush mechanics, pg. 164.

Pipe changes seldom make any more power. WHat they typically do if they are any good at all is shift power around in the power band. Typically more power on top as the better breathing lets it rev a bit more but with a similar loss in low end torque.