Very. Basically, if you want a custom exhaust made to the highest standard and to your specs and you live down this way, then you go to H&S. I would imagine that £650+VAT fitted is the discount price for being the first one, as that's silly cheap for a true dual exhaust to Rich's spec.
True dual will give a slightly different tone to a standard system Rich, but noise depends on what size silencers they use. If they propose on doing it the way that Jag has been done, then you can expect it to be roughly as loud as my K1 I would imagine. Best thing to do is to ask them and get a rough dB guide from them. After all, this can't be the first time they've ever built a true dual exhaust for a V6.