Why not a supercharger? I know they are relatively expensive, but I believe that would be money better spent than buying Nitrous. NOS bottle empty - your slow again, Supercharger gets great gas mileage when driven nice, but the power is only a little bit of throttle away.
You can check out one supercharger in particular here:
http://www.pt-tuning.com/performance...r_Whipple.html
I would eventually like to get a S/C for my 2006 convertible, but it only has 400 miles on it right now, and I have another 35,000 miles of warranty, so I can't do anything that dramatic yet.
Hope that helps,
Matt