I've just gone through a similar thing with mine. I have a Japspeed K2 and although it sounded 'nice' I wanted a bit more volume.
I had a new centre section made up last weekend (from the Y pipe to the rear silencer - check my build thread). The OEM one is 2.25 inches in diameter with a muffler in the middle. The new centre section I have is a 3 inch diameter straight thru without the muffler. Because the rear silencer (in my case a K2, but also the Milltek) is chambered, it hasn't really increased the volume by much. A little, yes, and it does sound nice in the car at 2k rpm, but volume wise, not a massive difference.
I'd go for the HFC if I were you.