I don't have PI heads to compare to, but I do have a PI intake and PI cams combo in my NPI headed stang. I put on the PI cams first and recently added the PI intake for improved reliability and better performance and really like the way the car is performing now. The power doesn't fall off like it did pre PI parts.
Here are my estimates as I have yet to get re-dyno'd post PI intake (I do have post PI cams though in sig): I hope this is not ricer math, although my sotp says it is not too far off.
PI intake +15rwhp
PI cams +10rwhp
total 25rwhp
PI heads (non-ported and no LTs or shortys added) includes PI heads, PI intake and PI cams +35-40rwhp
So comparing just the NPI heads with PI intake to the PI heads/cams/intake, you would be seeing a 20-25 rwhp gain. Not bad, but you also have to also factor in the increased compression that adds some of the hp and would require putting in premium gas. I'm anxious to see how my combo fares in the dyno compared to what PI headswap people are seeing.