I has a Milltek on an old R53 Mini Cooper.
A touch more peak power, but a tiny fraction less elsewhere - thats the nature of exhaust mods innisolation. Lovely fit, deeper tone but no louder than stock, and 22 years on its still going strong (a friends Dad bought it so I get occasional reports.)
Based on my experience there I'd very happily have a Milltek on my van if even I needed a new pipe.