Aluminum is fixable, and at these prices it's worth fixing (was it Mickey Thompson who said "anything aluminum can be fixed - if you don't care how long it takes, or what it looks like afterward"?). I don't know of cooling differences in the RB block.
I'm sure this doesn't apply to every engine, but there appears to be a cylinder pressure limit where aluminum begins to fail head gaskets, leak fluids, drop oil pressure etc. because of dimensional limitations (you can't just add metal or nothing OEM will fit), but who knows where this is?
My Toyota 183" L6 factory iron block & aluminum head can take 200 hp per cylinder with nothing special except fasteners. I would feel more confident with iron, but that's just my OCD.
Let us know what you're thinking?