If you're in beta, you know that shadow priests are fine.
Dispersion is 36% of your mana pool every 3 minutes for 6 seconds. If dots are "falling" in that time, you're doing something wrong.
Mind flay refreshes SWP. You can cast SWP once in a fight and it stays up the entire fight. Now imagine 2 or more simultaneous targets...
5% more damage to twin disciplines.
Haste and hit are common stats on crappy items now. You see it very frequently in the expansion, and have no problem hitting sweet spots.
Shadow priests will be desirable for 5s, 10s and 25s simply due to misery and the vamp embrace. In fact I see with the current damage increase we're seeing, VE is once again very strong and may in fact end up nerfed. Especially since our -aggro talents have been buffed.
You're also forgetting that you can only drink a pot ONCE PER FIGHT, where currently many classes still chug potions on top of spriests. Other classes getting some still yields an overall loss of mana regen because of potion nerf.
On top of all this, we have an inner focused' mind sear every 3 minutes for when you're dealing with swarms and non-ccable multi-pulls.
The DPS gain is major in WOTLK and we're much stronger at, say, 72 relative to other classes than we are currently. Of course once the itemization sinks in and our scaling kicks in it will be the same problem that it was pre-TBC as well as TBC.