"tune tune tune" will not get you better MPG out of a cam setup.... But it is possible if you know the correct tricks to boost MPG. Overall the cam/headers will probably help you get a mpg or 2 better on the highway, but you will probably get less in town due to the nature of horsepower....
I would probably keep the HPT, and toss the rest of the parts up for sale personally... I have 2 stock GTP's and I build/own some of the fastest 3800s on the planet.... And I still think my stock GTP's are fast enough to drive every day, race around stop lights, etc.... They are quite fun to drive. I was able to tune in a good amount of power (10whp maybe) and significantly bump MPG on my GTPs by tweaking some tuning bits.
If "beating that mustang" is worth it to you, then by all means toss the cam and headers in... but realize you will be basically throwing money away at that point, while right now you stand a good chance of recouping good money on your trunk o parts.
