Mike et al:
Time to end the "what ifs'", and turn the key on this puppy.
I've got 10 bucks that says his 300 watt load will stand 1500 watts CW dits and das for the 15 seconds. I would follow that up of course with a minute or two cool down.
Now, if you have a muffin fan or similar to run across it at the time, so much the better!
Don