Well, most of the time if the pipe is made for a .21 engine I'd say it will work with the .25. In my experience, pipes seem to have a fairly wide range of engines they work with, and usually it's grouped by block size. A pipe made for the TRX 2.5 will also work with the TRX 3.3. My THS Savage pipe on my LRP works for .25-.28 sized engines. If it seems to be holding it back, you could bore out the stinger a mm or two with a drill. I've done that before with RTR pipes. I looked up the pipe for the GX .21, and it looks close to the same size as the one that comes with the KE .25, so you should be OK. You could always get the .25 pipe later if the original isn't working well with it, or find an aftermarket buggy pipe. You might be able to luck out and find one for cheap or even one with the engine on one of the Ebay chop shops. guys buy an RTR truck and part it out for a profit. I've seen that pipe with header and springs go for as low as $16 US before. I get most my parts for the Mad Force off those chop shops.

Edit: I looked at my parts list for the Mad Force, and the pipe the new Mad Force uses is the same as the one used on the KE .25 powered Inferno buggy and is also used in the FO-XX (Same basic truck as the Mad Force), #IF242B. So You could look for those vehicles parts on Ebay as well.