Compression ratio doesn't care what the head is made of. There is no way the the cooling system can pull the heat away fast enough to make a difference. Aluminum may (and does) conduct heat away quicker than cast iron, but the only way it can carry away that heat is through the cooling system.
I not going to to the math, but how many BTU's will it take to raise the temperature of the coolant 1 degree? I'd say it's a bunch.
When the system is relatively stable, and you run into detonation, the aluminum won't be able to dump that heat any quicker into the coolant than CI. At least not enough or quick enough to make a difference.
