In case this is of use to others here, this is the garage I would recommend for modding work.
I have used them for 15 years, working on my various Lotus and Ferrari. The garage works on classics and also race cars, as well as doing high performance mods e.g. engine swaps and supercharging. Fairly priced and honest too.
Ask for the owner Andy Marsh. Garage is Maidstone Sports Cars.
01622 890 233
http://www.maidstonesportscars.co.uk/
The exhaust sound mod will be a walk in the park for them. If a number of people of i8 owners want it, it might be worth asking the garage to bulk buy the kits & install for a discounted price.