For what ever you think it is worth here are some of my thoughts. As you have already seen the damage of a tail section that could not withstand the thrust of open throttle, does anybody think it is a good idea to hang a plane from a "divingboard and scale" to measure thrust? I would not do that with any size plane regardless of the size or source of power. I saw the method that NJ Georgee is refering to being used on a cd rom motor foamy and I don't think I would trust even that to be safe with only 12 oz. of thrust and 10oz. of aircraft weight.
I agree with kdheath that power is not always the answer when in trouble. I have been teaching rc flying for ove 15 years and still believe proper control surface commands work better than brute power. I teach novices at no higher than 1/2 throttle untill they can land consistantly.
the_plumber is right about it's better to learn that the tail section was not up to the task while you were still on the ground. If the plane had been in the air when the tail let go the plane would be toast and someone could have been hurt due to the lack of control. Thank what ever Great Powers you believe in that no one was in front of the plane when this accident happened.
I hope you can make proper repairs soon and get back to learning to fly. As one of the guys pointed out, be sure to rebalance after the repairs
I'm not saying your instructor is wrong about wanting more power. Just remember that the Sig Kadet series of plane are a design that has a high ratio of design drag. I have had a lot of time with the Kadet Junior, MkII and the Senior and all of them are a great trainer BUT are not fast planes. They are made to float along and give you plenty of time to figure out your attitude condition and make a correction.
Good luck with your learning to fly. I hope you learn to be both a Great Pilot and a Great Builder.