It is not too bad IF you can get the airflow of your heads and enter your camshaft.
Dyno Sim (the new one) seems to have much better tools and has flow files for some aftermarket heads (AFR/ED) as well as many camshaft profiles. It is comparitively VERY expensive though.
I think they are both a bit optimistic, but you can use the information as one part of your decision.
FWIW