Here's my results after replacing my '08 wr450f stock header pipe with a '06 - '08 YZF450 header pipe, along with a PMB tip. All "free" mods/uncorking were already performed prior to replacement of these two items. The low end torque was definitely increased. At low rpm's where the engine used to chatter, the engine now pulls well. I did not notice any increase in mid range or top end. The PMB exhaust
noise is only slightly more than the stock tip with the pea shooter removed. At first I jetted the bike slightly richer with the YZ header, but after seeing some soot in the spark arrestor screen I went back to the 168 main jet and it seemed to run the same. In a nutshell, if you're looking for better top end this won't do it, but if low speed torque is important you'll like the improvement. I don't want more noise so that is why I did not replace the entire stock muffler assembly.