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
Think you have not spotted the OP is asking about a 370 system as I have never heard of a 2nd hand Japspeed 370 exhaust being available...if indeed they make one for a 370?
Stillen has worked well for me in the 4.5 years it has been on my car and still brings out the s each time I turn the key.
I'll just grab my coat