The old adage that aluminum heads effect compression is an old wives tale. Couldn't be further from the truth. 10:1 with iron heads is 10:1 with aluminum heads or 10:1 with wood heads. All things equal, cylinder pressure will be the same regardless, so compression is totally and completely unaffected. Where the difference lies is Iron tends to hold heat in, while aluminum tends to dissipate it. That extra heat dissipation does absolutely not one thing to reduce compression. Nothing.All it does is allow the combustion process to run a little cooler. Nothing more. That is the only reason you can get away with more compression using aluminum heads. That's why an iron headed motor will spark knock before an aluminum headed motor.
