You should get a very large change in sound once the DP is done. But that obviously also has a large change in performance too. It's a shame you haven't done the DP first so you can compare the noise levels with the stock cat-back vs the Milltek one!
Even with the stock-cat-back, if I suddenly go WOT, I've had cars in front of me swerve - presumably cause I've scared the **** out of them with the volume. I'd be nice if there was a cat-back system which actually reduced the peak volume! :)