I'll bite.
I was of the first people in the UK, possibly the first, to fit a Rotrex, back 10 years ago. It was fitted to a Mazda
If the kit has been worked out properly they are nice things, the installation is potentially simple and I am sure you have read up on the performance differences of a supercharger vs. a turbo. Power delivery is more linear and the known quantity of air makes for easier mapping. You do however get parasitic losses over a turbo but I very much doubt that would affect your fuel economy over a turbo. The issue in how much fuel you use is how much you press the pedal. Use your cruse control and I doubt you would notice the difference. In practice however you are going to see an increase in fuel use, because you will use it more.
The $64,000 question is is it worth it? That very much depends on what you want and what you expect and obviously how much it costs. I assume you are looking a number of thousands of pounds.
These cars are very much in my view the last of the line of the traditional Hot Hatch. A hark back to the Life on Mars and Ashes to Ashes mentality, made from recycled bean tins and cardboard, looked loud, was noisy and got airborne on bumps. When you could do a hundred on an empty motorway and smash down an A road overtaking Cortina's and Morris Ital's and very few people got upset. These days a GTi has adaptive cruse control, electric moving seats, brake assist, active damping, WiFi, launch control, lane assist and all the other cobblers.
These cars are still fantastic at blasting down back roads and A roads at smile inducing antisocial speed and in that it has plenty enough go. It is still however going to be a Clio, made of cheese and Gauloises wrappers and it is never going to be a Golf R even with 300 horses.
Personally, I think the joy is in the chassis and for that and the roads it loves, it has more than enough go. If you live in dual carriageway central and taking it to a 330D is your thing, then you might want to spend the money. But I would suggest that with the money you are likely to spend and the value of your car you could be in something that has it already.
