Not that I don't believe you but I'd really like to see dyno graphs to back that up. Not being a smart ass here, actually being serious - do you by any chance have any? I've often wondered just how much difference, if any I'd see.
ARC coolers are rad, I had one (and still have a damaged but air tight one for an R34 GTT in my roof). Not sure what you mean about legal, unless you mean the factory mount replacements, which yeah - you'd get away with.