LOL i dont know if you know, but me and zenith designed the 2c and kp sites, thats why they look the same, kp has separated from 2c nowdays, he owns the interlectural rights to the designs.
Onto your question, its hard to answer, in some cases the kp pipes out perform the fmf but the factory fatty is a good pipe, no pipes do much on a cr125-its the design of the engine, as much as i would like you to run a kp pipe i think it will be too much hassle and therefore cost to get one, you could try
www.1stmx.co.uk as they were selling off the old 2c stock for £120 complete , thats about $200.