Possible attack against a connected-mode pool: when you find a pool-grade hash, send it immediately to gain credit, but when you find a block-grade hash, hold it until the pool hashrate dips.
This introduces some risk that someone outside the pool will find a block in the meantime, so you can't hold too long, but the optimal wait time is almost certainly greater than zero.
Probably not. I didn't do exact math around it, but though about this possibility, too. You cannot take the share too long, because with new bitcoin block, your share become invalid. And this time should be proportional potential gain of delayed submit... Of course I'll welcome exact calculation about this possibility.