i have a 1991 250x that beats the 300exs the rear fenders from a 400ex will fit but the front will not without major modification

 

The 400ex exhaust (not with header pipe) allows more flow for the 250x, as far as more flow than an aftermarket exhaust I have not compared them but there is plenty of flow at a decent decible level. You more than likely will have to make some modifications to fit your headpipe. As far as the 400ex plastic I have seen 250X's with them but I do not know what it took to get them on.