You may be able to make a little more by mining @ a pps pool during the beginning of slush's rounds, then coming on at the back end of a round on slush to get your reward for that block.
BUT, you won't be taking extra profits from everyone else @ slush. You will still be getting your fair share. As opposed to mining only right at the beginning of rounds, when you find a very short round, you actually make more than you normally would, and take a percentage of the share from the other miners on slush.
I could be way off base here, but that's how I interpreted it...
So you are saying a miner working for an hour before a block is found is not actually doing anything towards finding the block?
You are contributing little work but getting a big share from the block. The block has the same value no matter how many miners are involved, so those who have been mining continually with slush have to lose out!
Yes, you're viewing a round as one monolithic problem solved by the pool for the network. Except the problem solved is extending a chain, so everytime a block is found on the network the problem changes. It's all a race with randomness thrown in.