Putting an issue aside as if they were a bolt in and go, I would have to go with weight.
Making the most of the available stock or ported head flow is the biggest key in power production. The poly head idea is excellent but this particular head may not flow the best or be ported out as well as the current available after market heads will flow.
There is a fella on the sister site, FBBO, that has taken a lot of time on poly heads to come up with something really nice. Not for the average guy kind of stuff. Milled rocker stands, he took molds of the chamber for custom pistons, etc....
Click the link -->
http://www.forbbodiesonly.com/mopar...-CNC-Ported-318-Poly-Heads&highlight=CNC+poly