Definitely not a waste. You still get more HP but not twice the gains for not quite double the price. I just think the symmetry of the DCAI is awesome and you do get a few more hp. Also, this thing sucks a lot of air and it sounds awesome. Always a personal choice but it was my choice and I'm not sorry for it
The double gets you 6 extra hp for $400, a EuroCharged tune nets around 18 hp at the wheels for that $400. Seems to me that for your $800 you get more for your dollar with a single. All figures are from claims and may or not have been substantiated. The gains are not cumulative anyway.
My car runs with the NW SCAI, NW coolant separation, NW oil catchcan and the Koni adjustable shocks from NW, these can make the ride softer than the NA if you want.