Maybe you are just ****** and don't realise but the ability of Rabadon's Deathcap doesn't stack... UNIQUE means no matter how many of the item you have you only get the effect ONCE. So you need 770 AP before the effect of -one- of your Rabadon's will take it 1001.

As for constructive criticism, Karthus' wall is extremely useful in Dominion, especially when it stretches across the lane, so it is worth getting another point in it earlier than you have.

Anything else I'd have to try it to be sure so I'll leave it at that, though it seems as if there could be mana or survivability issues if not used carefully.