The full math is here - David you would probably be interested in this if you haven't already seen it.

http://www.bitcoinunlimited.info/resources/1txn.pdf

The paper also describes how the sigops attack is mitigated through miners simply mining 1tx blocks whilst validating then pushing that out to other miners whilst they are still validating the 'poison' block. Rational miners will validate the smaller block, and they also be able to mine another block on top of this, orphaning the poison block.

The attacker would get one shot, and would quickly be shut out. If you have enough hash rate to be mining blocks yourself its really much more profitable to behave!