There's alot you can do to improve your dps. Don't listen to naysayers about shadow dps. Since 3.3 went live, spriests got huge boosts to dps through haste. First, visit shadowpriest.com. Best raid gear and max dps threads are a must to read and will tell you how to gear and maximize your spell rotations for the best dps.
Haste, haste, Haste. I cannot say it enough. Haste is like crack for us and the more you have the better. VT and DP now scale with haste and there is no known cap thus far, so you must have as much haste as you can get. Shoot for getting 1k haste while maintaining 22% or so crit and staying hit capped. Staying hit capped can be tricky at the higher haste levels. Gem for haste wherever you can - reckless ametrines in ALL yellow sockets. Ignore socket bonuses that do not give you at least 7SP and use runed cardinal rubies instead. Use chaotic skyflare diamond for the meta and activate it with purified dreadstones.
Avoid gear with spirit on it as it really doesn't help us much. Did I mention haste? oh yeah, take ANY piece you can get that has haste on it. Abyssal rune from reg ToC is a great little trinket for us until you can get something better and it is easy to get. Eat haste crack regularly - made from imerial manta ray fish. Potions of wild magic are awesome, prepot before the pull so you can use two of them during the fight. Pop any 'on use' trinkets and potions before you apply sw:pain.
This spec takes a skilled player. It is not for the 2-3 button mashing type player. It takes alot of awareness of DoT uptime. Yes, you actually need to think about what you're doing which makes it challenging. I recommend using Quartz which is an add on that will help you keep your DoT's up and will also show you the ticks on your mind flays so you can make sure you aren't clipping. It's all about timing and keeping your DoT's rolling. Ideally, you want 90+ uptime on your DoT's to be effective.
Unbuffed, I'm sitting at 964 haste and when I eat my haste crack I'm over 1k haste and doing some pretty impressive dps. Fully buffed and flasked, I'm sitting at around 3300 spellpower and 25% crit. I'll never beat the rogues and mages - rogues and mages are pures and we should not beat them on the meters, but I am ususally just behind them, within the top 5-6 in dps in our 25 man raids (ret pallies are OP and need to be nerfed lol). I'm still missing a few key pieces of gear as well, so can only improve once I get the drops. Dispersion is your friend in ICC, there are so many fights that it becomes useful and will allow you to mitigate damage instead of having to stop casting, move or let stacks of debuffs fall off. Once I got 4pc T10, I dropped Improved Mind Blast along with the glyph and glyphed for Dispersion and spec'd into Improved VE and Silence for PvP. Improved VE heals for a butt ton and makes our survivability that much better.
Don't be discouraged, it takes skill and a ton of research to play this spec. Spriests with low dps aren't geared nor playing properly. Visit shadowpriest.com for more information. It is an excellent resource. Hope this helps you, good luck!