Seeing that the taunt has a chance of killing you outright, perhaps this could work?

On win:
+(5 rounds) Gain 5 seconds of critical hits
+(4 or 3 rounds) Gain 5 seconds of minicrits

On lose: Self-explanatory

-Positive bonuses only take effect if enemies are within medium range*

*See any weapon on official TF wiki for Valve's definition of medium range