I don't have enough stats to say how much effect this really has... With two workers that have the same hashrate you would think that the pool server would give out shares of the same difficulty to each, but that is definitely not the case with slush's pool or the stratum-mining pool software I've been playing with. More info on the pool server software:
https://bitcointalk.org/index.php?topic=162788.0Thanks Stew - looks like it's helping me get another 5% since I had so many cubes sharing the same proxy. Definitely appreciated!