Great thread! Here is my 2 cents. There is more to think about than just HP per $.
With LTs, MSD, Rotofab, and a good tune for $3-4K my M6 SS a solid 11 second car with a decent 60'. All that adds around 35-40 HP in parts of the curve but moves power somewhat upwards towards the redline. A little less fun on the street. Without the MSD you are not going to get to those numbers. And in a mostly stock car exhaust adds nothing you will notice.
I think with a less restrictive air filter, a ported TB (just for fun), a ported manifold, E85, and a good tune my SS would be in the same place but with less of a move toward the upper RPM for less than 1/2 the cost, especially if you DIY the parts yourself and get a remote tune. And if you add HP Tuners and DIY the parts I would think you are still under $2K.
Check this out, an E86 M6 SS with a solid 11.8:
http://www.camaro6.com/forums/showthread.php?t=475780
If you think about the future, and with modding you should, it would be easy to put the E85 car back to stock if you want something else. Nobody and especially dealers want a modified car. If you sell it modified be prepared to take a bath with losing additional $$ when selling the car.
As for me the A10 is weakening my no autos resolve. It will be a hassle for me to put my car back to stock to get something with the A10. Life is for learning.