re: I've just spent two weeks in the Palmetto State

Your description of both fanbases is closer to reality than most would care to admit. As for the split in Greenville, I'd say you are correct. Once you get down to Newberry County, the split becomes overwhelmingly Gamecock all the way to Sumter County. Then the split becomes more even until you get to the coast, which is more Gamecock than Tiger, especially outside Charleston. Horry and Georgetown Counties are 2 to 1 Gamecocks fans.