I figured I'll post each of my mod questions individually in case someone knows about one but not the other. Anyway, since I have this stock 5.0 I was thinking of going the usual heads/cam/headers/exhaust/etc. route. I haven't been able to find much information about gains experienced with a set of Trick Flow heads on these cars. How much can I expect with an otherwise stock car, and how much can I expect with a modified (cam, headers, full exhaust, etc.) car?

Depends on your combo and which heads you are talking about.