I too am having an issue with under-performing units. The fastest is 2.1 GH/s the others are all below 2 GH/s Hopefully a spare unit or firmware update will fix these! Also what's happening with the compensation for delays? Has that been announced yet?
I don't think the probability of winning or losing is influenced by them playing their own game (If they even are the ones playing it) as all the seeds are pre-generated that they use to calculate the hash and determine win/lose. You can download the hashes of the seedlist here http://www.satoshidice.com/static/hash.keys to verify the seed used on their released list here http://www.satoshidice.com/secretlist.php is indeed the seed they said they used.