i can answer that. that will most definatly damage those motors. those motors are wound for 6v, and because these things sit in ground effect, they get nearly no effective cooling. good news is these motors are cheap, so you could probably put heat sinks on to help them last a bit, and then after maybe a half hour of runtime, replace all four motors and check to see how badly worn the motors are. look for wear on the brushes, and you can gauge about how long you can go before replacing motors. this is important because brushed motors will sometimes wear differently and cause un-balanced responses, one motor to produce more thrust than the other, etc. NEVER REPLACE JUST ONE MOTOR AT A TIME. it will be like just replacing one tire on your car. if you replace one, replace them all.

you may also want to consider re-timing your motors. if i am not mistaken,the 350c motors are being run backwards for our application. i think ADI had an article on how to retime those motors, but if you timed them correctly and tested it to make sure it is good, you may be able to increase your power output and reduce your heat output significantly.