This really isn't true at all. I'm not talking from a lack of experience either. I've finished the game twice as a Sorcery focused build, and twice as a Pyromancy focused build. The Pryomancy build is so much more consistent throughout the entire game, and you don't have to wait until the game is almost over (which so many people keep ignoring). Sorcery does not "get good" until around Anor Londo, and you're still going to just be using the basic Soul Arrow, Great Soul Arrow and Great Heavy Soul Arrow (which has a terrible casting time, even at the casting cap).
I'm sorry, but 700+ damage for 22 mana with a much greater casting speed is better in this game any day, all day over something that costs 15 FP and won't finish casting the second one you need to actually kill off an enemy unless you cast the first one at max range, and the enemy doesn't happen to have a shield that blocks the second.
You also get to use a shield, and don't have to use an off hand item that requires FTH to boost your damage slightly, nor do you have to wear the dumbest looking helm ever created. Sorcery requires two more slots to do higher damage. Using equal slots, it does close to the same, is more efficient per cast, but much, much slower overall, and there are very few enemies weak against magic aside from specific knights, and a lot that are weak against fire. There's just not many enemies that are actually highly resistant to magic in this, and there's like three (?) mini bosses that have fire resistance, as well as an optional area that's filled with marginally resistant enemies. I can't even think of more than a single actual boss that has much of a resistance to fire either. And for that, you have another spell that's not fire based anyway. INT is a part of Pyromancy anyway, so it's kind of redundant to talk about Dark spells as though they are somehow a Sorcery exclusive. Those that have no FTH requirement anyway are terrible in both PvE and PvP too.
I don't know where you're getting the 850 Vestiges damage either. At 30/30 it hits for over 900, and against enemies with a weakness to fire, it can hit for 1200+.