From what I can tell, the diff on a modern FE iron casting and aluminium is about 30 hp. The diff between an OEM block and aluminium is 15-20 hp. We have done several engines that have identical or nearly identical parts, and those are the margins that show up, over and over. The Ford OEM 427 is not as much better, but the Pond, Gen, BBM iron blocks are "more" better, just because of the more rigid cylinders. As far as "in car" goes, you can chase a dyno figure, and an iron block will show more, but the power is just about exactly offset by the weight difference. It takes about 30 horsepower to overcome the 150 lbs. In a drag car, 150 lbs off the nose is ALWAYS better. The chassis can be better optimized with less weight on the nose..........always. I remember when we were allowed to replace the OEM seats in Super Stock with lightweight race seats. Even at the same total weight, my car picked up about .03 in the 330 ft time from taking 80 lbs out of the middle of the car and relocating the weight to the rear. That was in the middle of the car, and about half the diff in the iron and alum blocks.....
To Barry's point on aluminium....I totally agree on the growth. I always run the deck a little tighter, and also bump the static C/R a little, to allow for the growth. The last Shelby block we did had .008 cold lash, and about .025 at running temps. They definitely grow more. As long as you know that, and allow for it, it is no problem. Aluminium engines just need to be massaged a little different than the iron.
If you are traction-limited, or nose-heavy, the aluminium block offers added benefits to getting the car to hook and "work". Small tires and/or leaf sprung Cal-Trac cars with big engines will like the benefits of the lighter block more than they like the power diff of the iron and the weight it puts on the front tires. A tenth gained in the 330 ft is a "mile" in high gear..............