what pitch does the tail go to, i.e. is your problem a set up problem, when your heli spins to the left is that the same way as the blades go! if it is it's probley the setup of the tail, you need to move your servo, or trim, I doubt it's the servo not have enough power,
and if it spinning the same way as the blades the tail rotor is over compensating so there more than enough power in the tail rotor and the servo,
I don't think it was not such a good idea to put 450 blades on a 400, there not built for a larger blade,