Who cares what the mpg would be? Not everyone likes boosted cars & also something like this would be very unique, especially in the UK.
Also the cost of it won't be a lot more than doing a supercharger properly & a hell of a lot less than a TT setup - combine that with the fact that the LS block will handle boost better than a stock VQ & I know which one I would prefer
I'm still waiting to for someone to pull the trigger on a RB26/30 converision