There's not much you can do about shadow priests low dps tbh. Shadow Priests are currently the lowest damage class in game, they're used rather for their 3% raid wide hit buff and Replenishment. If you are making a shadowpriest you want absolutely no haste procs from anything what so ever. Haste procs will screw your rotation up if procing at random. The stats you want to aim for as a Spriest, bout 25% crit 800 haste, and as much spell power as you can possibly get in after that. My Spriest set isn't in much more than 245s / 251s and I pull about 5k which isn't very good either. Good spriests can pull a substantial amount of dps but they're never going to be anywhere close to the 12-15k mages can pull on some fights. They're used for raid Utility rather.
If you're having problems with your rotation, just make sure you've always got VT up, always have DP up, and as soon as Mind Blast / Shadow Word: Death are off CD you should be using them immidiately, throwing mind flays inbetween. Flasks - Frost Wyrm / Potions - Wild Magic