The parmakit is pretty awesome, but like nick said requires some messing with. It helps to have access to machine shop equipment, but it isn't really necessary. You basically put in some extra base gaskets so the piston clears the exhaust port and then remove a little bit of material from the top of the cylinder to make up the difference. You can do this by holding the kit top down on some fine grit sandpaper fixed to a flat surface of some sort (piece of glass) and sanding in a figure 8 motion. It'll take forever, but it can be done without fancy tools.
That said when my parmakit blew up I went for the 47mm Airsal. From everything I read performance was comparable and it didn't have the core shift defect. Still tuning so I don't have the full verdict yet.
As far as fuel economy goes to me it's more about how the engine as a whole utilizes the fuel rather then raw displacement. Having a high performance 50cc kit with big ports and a large carburetor will get significantly lower mpg then a stock setup as well.