Actual real world tests show very very little if any power loss from iron vs aluminum. I have seen several dyno tests of heads in iron and alumnium even as can be like CNC ported and there was no real world differance in the power, 2 or 3 tops on a 350 chevy. The aluminum head allows more compression, it does NOT require more compression.

As for your combo feeling weeker it could be a few things, mabey timing was not optimised for the new heads(they will need less timing) and the carb could be running lean with the extra airflow. Mabey the car is hooking better do to weight so now you can not roast the tires like you were. Mabey the tq converter needs adjusted also. Several things to look at before jumping to blame the heads. If there is a problem with the heads it would show up in a leakdown test(bad valve job).