They are as.good.as 906 heads the new stainless valves will help reliability and flow the most( one piece and already back cut) yes you will end up with probably 700 in your heads and aluminums are double that, with aluminum you need to increase comp ratio to stay at same power level as iron due to the aluminum heads losing heat quickly. So if your sticking with basically stock replacement on bottom end and a relatively low lift cam (under .500 lift) you will lose power with aluminum heads.
It is unfortunate but most of this information on aluminum heads is false. Seach as I may, I cannot find any facts backing up these continually perpetuated myths. Below are two articles where this horsepower loss was tested and found to be not so. So surprised were some of the testers, they denied their own findings and surmised they must have done something wrong. They
were wrong.....in that they didn't believe their own results.
If you can afford the aluminum heads, by all means buy them. Even at lower compression ratios you will get an increase in power. But be advised, all aftermarket heads should be properly prepared by a knowledgeable cylinderhead technician. Too many aftermarket heads have valve guides too tight, seats out of round and sealing surfaces that are not flat.
I have found the 440 Source valve springs to be consistently closer in pressures than the Edelbrock RPM springs. The 440 Source valve locks and the assembly procedures result in damage to the valve stems below and at the locking groove and I've had to replace many 440 Source valves when I dis-assembled them to prep them.
Any of the aftermarket aluminum heads will out flow a rebuilt factory iron head unless your technician is very proficient in porting iron heads. You can't just slap in bigger stainless valves and call it good. You may get some improvement but it will be nothing compared to a prepped aluminum head.
The early Stealth heads had a problem with the gaskets overhanging the combustion chamber. The later heads have had the combustion chamber reduced in size (from 84 to 80 cc's) and if you use the FelPro 8519 PT1 head gasket it shouldn't be a problem.
As to the aluminum weight difference,
any reduction in weight in our heavy C-Bodies would be beneficial in handling, fuel economy, acceleration and braking.
http://www.carcraft.com/techarticles/ccrp_0602_iron_versus_aluminum_cylinder_heads_test/viewall.html
http://www.chevyhiperformance.com/techarticles/83858_iron_vs_alloy_engine_heads/