Remember how I said proline makes them for monster trucks? So how many monster trucks do you see with them on there at the race track... NONE. Proline makes some of the best products there are and have great recources for racing and development but they didn't get your system to work on a race track. Yes, those were for monster trucks but still its not a new idea and if top manufactures or other aftermarket companies don't have them yet then it means they give you no advantage over the stock configuration and the two springs only make things more complicated.
you are wrong, i had them, they worked great, almost handled as well as a revo.
and again its different for mosters, they are heavier alowing that to work. for this app, i would recommend either just sticking with standered coils, or get the pre made progressive.