I totally agree with the first sentence; second sentence is true as well but the mistake they made was when they went with the lighter casting they disrupted the possibility of bigger valves without affecting velocity in the aluminum design. you can affectively put 2.02-1.6's in the iron head, but cannot in the aluminum. Why you ask? In order to shave weight they put less metal so when you attempt to port you punch a hole into the pushrod area So when you TRY to put 2.02-1.6's in the LT1 stock aluminum head you overlap the valves and float them quicker. Manufacturers like Trick Flow and Edelbrock tried to overcome it but when you deal with our reverse cooling design it is rather difficult to achieve.

 

Correction it does not allow more compression it requires it due to the thermal efficiency. Back to a comment made earlier if aluminum moves heat faster than therefor you say the chamber will be cooler for the incomming charge, I would say that a bit of the heat being taken out of the chamber is being pumped into the charge while it is in the intake port and would bring more heat up to the intake manifold.

I know it sounds like I am just trying to be argumentative but the misconception that aluminum is magically better and iron has no place on a performance engine is a pet peeve of mine. Both sets of heads are good and swapping either stock for stock would be a complete waste of effort and money. Also I am comming from the side of this that got the irons stock, it doesn't happen much anymore but we used to get a lot of guys that would post how they got Z28 or Vette heads and were going to swap them in stock because they came from a Vette of F-body and that had to be where all the extra power came from. End result was a 30lbs. weight savings on a 4200lbs. car and depending on the casting of heads anything from a slight power lose to just equal power. Now tell me is a 30lbs. savings on a car that heavy worth the cost and effort of a head swap, better off to run a lighter battery and yank the AIR sytem(we can legally defeat it so most of us go the extra step and pull it) to shave that little weight.