my motor is going to start sputtering and stall before i overheat to the point where my piston welds itself to the cylinder wall correct?

 

no, probably not. If you overheat it enough, it will seize up.

 

but i'm sure power decrease will be so great before that happens that you will know to stop right?

 

when the motor gets hot you will notice a loss in power, but not a huge loss. When you hear it start knocking then you know the heat has started to thin out the oil and you'll start to hear more engine noise, when this happens you'll want to let it cool. I you continue riding long enough and it overheates bad enough then you could seize it up. since 400EX's are air cooled they heat up a little faster than liquid cooled engines do, you will really start to notice this in the sand.