I can't tell you how many times I've been asked about recommendations about putting a cam in a Magnum with stock heads. Everyone wants that rumpity rumpity sound, without the expense of buying heads too.
I had a 5.9 Jeep go dyno the other night. Huge cam (230/236 110 LSA) on stock heads, airgap, bolt ons.No cat, mids, 3" pipe. Dyno reads 10-20 whp low. 261 whp/320 wtq off the street, no tuning on Dyno. I might be able to get a few more out of it but not 50+.
In this particular case, the by the time that cam is ready to sing, the heads and to a lesser degree the intake manifold are done. So he has marginal low end, with less than stellar top end. Overall, NOT a good combo. I have trucks with M1s and bolt ons that make similiar if not better power. Imagine how disappointed that owner must feel. I feel bad too, but I can't get around airflow limitations.
Just my .02 on this rainy 9-11 here in Houston.