I believe the White Brothers cam is just a re-branded Webcam. I'm not sure what grind it would be though, that's where the big difference lies. It's probably just a .390, which will give you torque, but that's not what you need for racing, you'd be better off with a .430. Keep in mind though that on top of whatever you pay for the cam, you'll also have to get stiffer valve springs too for anything other than a hotcam, which is a mild piece of crap.
And with a .20 overbore, that puts you to like 2 - 4 cc's bigger than stock. They're not going to put you in a different class for being 2 cc's over the 350 mark.
With the right cam, a high compression piston, a good exhaust, open airbox with a clamp-on filter, and the jetting dialed in good you should be able to take a 400ex, at least a stock or piped one. Not sure about the z400 with that, it would probably be close enough that it would come down to rider skill.