does anyone know, and how?

According to most forums and dirtwheels the stock horse power is 27-28

NOT ENOUGH!!!!!!!

THOSE NUMBERS MAY BE CRANK HP ...MINE WITH A XR250r BOX AND A FULL YOSHIMURA PIPE AND JETTED MAKES 27.7 HP AT REAR WHEEL ...

Its 27HP in the rear wheels.

Well, really the only power-adding mod you have is the Yosh pipe (since the XR250 box just gets rid of the rev limiter), and depending on the type and brand of dyno, results can easily vary 10% or more. Most dyno runs show about 27hp on a stock EX at the rears, and I'm sure if you had yours dyno'd on one of those dynos, you would see more power than stock. Maybe not a much higher peak (1-2hp) but but your overall power could be much improved over stock. Just imagine having both a 2-stroke and 4-stroke that make 30hp peak. Which one has more power? Probably the 4-stroke because they generally have a much wider power curve, even though you have the same peak.