Oh and I forgot about answering the question of how and 8 man hydra and a two man hydra are different. It really isn't relevant to the debate but I'll answer it anyway.
Ok so as I explained earlier, Hydras are expected to play to the best of their ability. Part of this is actively conferring and checking with your partner. Doing this properly with one partner takes X amount of time and effort. Doing this with 7 partners takes 7x more time and effort on the part of each individual player and [8!] amount of effort for the play slot as a whole.
When we consider that people often play in hydras for reasons relating to inability to be sufficiently active to reliably maintain the activity necessary for a single player slot, your 8 man hydra becomes defeating of its own purpose.
If we assume perfect play from all the players involved in any given hydra, then yes, an 8 man hydra is the same as a 2 man hydra and is the same as a 9001 man hydra, However, practically speaking, as more people enter a hydra it becomes exponentially more difficult to play the hydra responsibly and well and ultimately defeats its purpose. Hence, the practical difference Tom was talking about.