Aluminum Heads ?

I'll add my take.. As far as strictly thermal efficiency "loss"? I'd say less than 1-2%. However, that loss can be offset very easily with design adjustments to the engine during the build. The only down sides really are the thermal loss, the added cooling system load, the corrosion issue, gasket choice, and in some cases lash issues from the expansion rates. Most of those can be countered with basic choices. More static ratio, better cooling system, the right type of gasket, and experience/different procedures. The up sides are wieght, thermal loss, modern port designs, easier (cost wise) porting, modern or at least closed chambers, and the bling factor. Changing heads, without accounting for or exploiting all of the benfits, will lead to disappointment if your package is well scienced out. That's why budget is so crutial ina build. That sets the type of head, and the head sets the stage for the power band.