This +1
If it was me I'd save the money on the HFC and either buy cheap set of De-Cats to test or Sell the Milltek and invest in a different exhaust, I feel you will be a bit disappointed still with HFC + Miltek if it already sounds quiet to you.
Options are Cobra, Custom (my route and love it!), invidia they are the most common, never hear complaints
Or you could pick up a 2nd hand Japspeed exhaust if you are after Noise, they can be found under £200