Why are so many people concerned with their top end speed/who is faster? Improve your handling, shift points, and your riding skills and you will be faster... Your friends Z400 is only as fast as you/he lets it go around the next bend, down the next set of whoops, off the next jump, around the next winding trail, and up the next hill climb. I ride with many people who have faster bikes then me, when it comes down to it I’m a much more skilled rider - guess who’s in front... I am saving for a YFZ for its jumping stability and light weight; my Warrior is still an excellent quad though and won’t be going anywhere. Him just having a faster bike then you doesn’t make him faster unless you let him; the fastest person (unless on straight shoot outs) is almost always the person with the biggest set, not the fastest machine. There are way to many penises around my area on fast quads and bikes that don’t have the nuts to keep up in anything but straight aways.
On another note a 446 FST stroker with the right setup will toast him.