I just weighed my 05 525 EXC yesterday with a bathroom scale. While putting the scale on the stand and the bike on the scale works in theory, it doesn't always work in practice. Ever been on a scale that you can alter the reading just by leaning to one side or the other? Well those scales are very likely to give a false reading with a motorcycle sitting on them and this was the case with mine. I resorted to putting the scale under each wheel and adding up the totals. I put a length of 2x4 wood across the scale so that the weight would be distributed more equally on the scale.
My bike has Probend handguards, a Flatland Racing aluminum skidplate, KTM hard parts case saver and rear brake rotor guard and is wearing a 150/90-18 Terra-Flex on the rear. With a full tank of fuel and the factory KTM DS hardware installed (turn signals, horn, combination switch) combined weight of front a rear was 282lbs. I think that considering the extra components my bike is shod with, that compares pretty good to the 276lbs that you have for the 07 450 EXC in your list.
Claimed dry weight for my bike is 251lbs. Not only would I have to remove all that extra stuff mentioned and drain all fluids from the bike and battery, but I would also have to remove the tires completely from the rims to make that weight!