All modifications are "not worth it" on a lot of cars simply because everyone has assumed the factory pieces are competent enough and no one has had the money or the courage to try something different and push the envelope a little. While there are trends that show that the stock heads and cams can support a lot of power on F/I setups, there is no data out there at least that we can find that would suggest that there are not worthwhile improvements to be had in an N/A setup concerning the head and cam package.
That sounds like the guy who buys a Honda because of the gas mileage, not the mustang owner looking for power.
As for the N/A heads part you stated, there are quite a few people who have changed jsut heads or just cams N/A and seen 12hp. Does the cost of the cams or heads plus labor justify 12hp? i think youd be hard pressed to find someone who will say yes unless they have unlimited money to spend
Im not arguing that heads and cams are a bad idea. I am using Livernois stage 3 heads and BBRStage 2 cams on my new motor, but i also did them because the lower CR ration combined with the Supercharger will net me gains much larger than if it were N/A. If i had stayed with the stock block, i doubt i would have done either at all because of the cost/hp gain ration is too high