Yes, probably the world expert on that is Bob Devlin at the Department of Fisheries and Oceans in West Vancouver. He has been very interested in growth hormone transgenic fish. By introducing a growth hormone into an animal, you can greatly accelerate the rate of development. But one of the really interesting things he has found is that rainbow trout have been domestically reared for over a century, and that just through animal husbandry there has been selection for the highest growers. So domesticated rainbow trout grow much more rapidly than wild rainbow trout, and through that husbandry, it seems that we've come more or less to a limit of how fast rainbow trout can grow. If you introduce transgene into wild rainbow trout, they don't get any bigger than those selected for fast growth through husbandry.
So I think there is an upper limit to growth that is just a physiological limit, and there is some understanding of that—although there's always a lot more to be learned.