How Fast?
Most of the replies you have gotten are fairly accurate. The top speed on a stock 400ex is 67-69. But my buddy bought one that we clocked with his dad's police radar gun and it runs 71. Some motors just come out of the shop that runs faster but my 400ex ran 68 when it was stock.
04 400ex( Duncan CDI Ignition, Ex-treme Racing lowering kit, wheelie bar, a-arms, 6+ swingarm, K&N(of course) Jet Kit, 86mm 10:1com piston, Hotcam stage 2, Fatboy4 full exhaust, nitrous kit, 16 tooth front sprocket, stock rear sprocket,HD valve springs, 8 plate clutch assembly) little bit more but not much