are these just your typical gt40 aluminum heads?
if so am i not mistaken that they don't really hold a candle to edelbrock, holley, trick flow, or afr heads?
gt-40 y's are good because they are aluminum, but other aluminum heads including other gt-40's, ford posts them as 20 hp less in their crate motors than gt-40 x's.
The site Mav provided gives lots of flow detail. PM me with you email address and I'll send along a spread sheet that someone put together that has data also. Ford makes the GT40Y, GT40X and I believe recently introduced the GT40Z aluminum heads. The Y's flow a bit better than the cast iron GT40's and GT40P's. The X's flow comparably on the intake side to a lot of the other street oriented alum. heads out there, but are a bit restricted on the exhaust side. The Z's were developed to allow FRPP to compete in the bigger flow arena.
So it depends on what you're trying to do. I got the Y's because I wanted the weight savings of aluminum, and I wanted something I was certain was dimensionally identical to the stock head. The flow/hp gains weren't that important to me. I'd say your average modder only picks up the Y's because they got a good deal on them, or because they intend to have them ported.