It's not madness as long as he isn't knackered from running in the St Leger but he may well have had something taken out of him. I recall Camelot running no sort of race in the Arc after he should have won the St L.
I honestly thought after HL won the group 1 in France, the arc was his no 1 target and was surprised to see a couple of weeks ago that they'd confirmed he would run in the St Leger. Obviously he went and won it so you can't knock it. It looks like a massive vote of confidence from Godolphin for Adayar.
My own personal opinion is Hurricane Lane is the better of the two, but Adayar has won the bigger races. Just have a feeling it might be a race too far for the hurricane.