I too think it's a great idea and has a very comicie feel to it. However, I too do think it needs to kick in earlier.
I'd suggest it being inversly related to the blasters health. at 95% health, 5% defiance buff. I also think the total amount of damage buff should be lower, so that a accuracy buff can be added to it.
So those final, desperate, shots really count when you need them to. A every increasing bonus to damage and accuracy would be very handy, while also being off set by the drain system already in place.