Just thought I would add my 0.02 to the whole "no difference between fermenting in a corny or in a carboy" conversation. I talked to my waste water treatment prof (damn you chemical engineering curriculum!) and we just covered a section on zone settling. I asked the prof after class what effect reducing the diameter of a fermentation vessel would have on the attenuation of the yeast, and my prof had a few things to say.
She pointed out that due to the similarity in the size of culture mid way through fermentation, the yeast inside the corny will flocculate sooner. Since the same number of yeast cells are confined to a smaller space, they hit critical density sooner, and will fall to the bottom of the fermentation vessel. Additionally, having a smaller diameter will reduce the surface area of wort that the yeast comes in contact with, which would prolong fermentation times. Needless to say, she made the point that she couldn't see why a corny keg would facilitate greater consumption of maltose by the yeast in these conditions.
This was all speculation on her part, but it seemed to make sense to me. This is purely qualitative, and I think someone will need to brew an 8 gallon batch, and separate the wort into a carboy and corny keg. The yeast would ideally be cultured so that we can be sure that they have the same viability. Finally, readings should be taken every couple of days so the progress for each can be tracked over the fermentation period (who know, maybe the corny ferments quicker, and flocculates sooner as well!).
I don't have a corny to spare, so I'm hoping someone brave out there is willing to do this experiement!
Cheers,
Jay