the flapper mod, thats no more than disabling the diaphragm on the back of the exhaust dude
its there for EU noise regulations to keep the car quiet at low revs, once you build the revs the diaphram opens and the car runs at normal noise levels, by disabling it you'll only get a minor increase in noise levels (same as when your at higher revs) so kinda pointless although will crack a small smile but no where near as large a smile as an aftermarket replacement imho
been there, done it