For your first question: just adding a big valve on a 906, without a bowl blend after opening the throat up, gets you nothing,except a bill at the machine shop. I've seen tests showing a big valve making the flow worse, from bad valve seat work.
In my humble opinion..... if you just had some old 906s that needed everything done to them, then no way would i put money into them with really good aluminum heads as an option.
But! You say they are fresh with seats. Decent springs? 906s on a 440 with a big cam and decent compression can get to 500 fairly easily. 550hp is much easier with good aluminum heads. Is that 50hp worth $1500-$2100 ? Thats for you to decide.
As an aside, i just put $900 into freshening up a set of iron heads..... but they were a beautifully full ported set of max wedge heads, no way i spend that on unported 906.