The exhaust flow is absurd... And they are only quoting the number's @.600. I would suspect they probably flowed them on a 4" bore too.
But simply going off the number's they look like nice head's. For myself, I will spend the dough and go with what I know.
If you want to build a badass modular, one that can eclipse 500 rwhp reliable, you will eclipse 10k in your engine buiild, period. Then you'll need a trans. If you want to go fast cheap and easy, you bought the wrong car,sell it and go get a LT! F-body.