Some available gains are impressive, I know the Saab turbo petrols respond well, as does the ubiquitous VAG 2.0 turbo petrol that's in pretty much everything these days.
I've always wondered about the merits of a plug in box Vs a 'proper' remap though. Clearly the former is more easily removed, but I wonder if it's as good for the engine.
Prior to the Viezu V-switch I purchased a tuning box from TDI Tuning which fitted under the bonnet. It cost around £350 and was utter crap, with no gains whatsoever. I sent it back for a refund. It was supposed to give an extra 60 BHP and 60 FT LB of torque.
........in reality it gave SFA.
As far as I am aware the V-Switch provides a new map (just like a proper remap) where a 'piggy back tuning box' simply disrupts the incoming signal (I think).
Car will now hit a genuine 100 MPH in less than 9 seconds (on a private road, obviously) according to my Racelogic GPS data logger.