hey how fast could my Z go with a yoshi, a jet kit, and k&n filter?

 

maybe 58-60mph

 

A pipe and air filter won't really increase your top speed. You will still have the same maximun RPM. To get a higer top speed you genarally change the gearing. In the case of the Z-250, all you can do is get larger tires.

 

wut size i could put them i got them stock front are 22-7-11 and rear are 20-10-9?

 

If it is stable enough for the riding you do now, I'd move up to a flatter 22inch. Stock tires tend to be too rounded, and won't handle as well, or hook up as well as a flatter profile tire.