That will work however the 250 Watt motor will run hot if more than 250 Watts (~1/3 horsepower) of load is applied to the motor. You could get away with momentarily loading the motor with more than a 250 watt load if the continuous load on the motor was 250 Watts or less. Volts and Watts do not harm motors, only excessive heat does, so as long as the 250 Watt motor was not overheated then it will run fine with a 36 Volt 800 Watt controller.
As a rule of thumb I always like to use a controller that is rated for the same or higher Wattage than the motor, but not the other way around. If you are overvolting a 24 Volt motor then its estimated Watts rating at the increased Voltage can be calculated using the tool at this link: http://electricscooterparts.com/motor-overvoltage-power-calculator.html