the short answer: it depends
the longer answer is: what is your ultimate intended goal? the alum heads probably have more room to grow, or will support a lot more power in out of the box form vs stock 906/915 heads. an aftermarket head is generally machined for larger valve springs, larger valves, hardened exhaust seats, updated valve guides and seals.
so basically, all of the stuff you'd need (and should) be doing to update a factory head for not only performance but also general use and longevity. then, add in standardized chambers and flow numbers that rival steel heads that have already been ported.
i'd bet dollars to donuts if you sat down and did the math, new alum heads would be darn near the same price as a set of full worked over 906's by the time the dust settled.
