...
Just to follow-up to my earlier post....
1) There is a possibility for mining in a pool, and withholding blocks, to be net profitable for an attacker solely by the effect on the difficulty level, and delaying the difficulty retargeting.
...
Have you read
my post some time ago?
Yes, it is possible, but it takes time (2+ months) to profit from it - to get the attacker and punish him for that is the 'antidote' for such attempts