The gem advice here isn't 100% correct, follow the gem advice at the top of
this thread.
I think its quite evident you haven't done this yet, I bet you haven't analysed any combat logs either or even bothered to break down a target dummy recount parse. Doing this is mentioned in the sticky I wrote.
I normally don't bother analysing peoples gear for these threads, the gear analysis is rather self explanatory if you bother to read the stickies and visit shadowpriest.com. In this case the gear isn't awful but you have far too much hit, a spriest hit cap is 11% as explained above which is 289 hit rating. With ToGC gear or ilvl 232/245 you can expect to possibly be well over the hit cap due to the vast amounts of hit on gear. However at the gear level you are currently at there is really no excuse for being so far over, it's wasted itemisation.
Never gem for haste, there are only really 3 gems worth considering, those being Purified Dreadstone, Runed Cardinal Ruby & Potent Ametrine. Use them per the link i provided earlier for gem advice.
Talent build is fine, I dare say due to the complexity of shadow dps that your rotation is far from ideal, you can only really check this via combat logs analysis. Basically your MB should be cast every 7.2 seconds, dots need to be up 95% of the time and you should get a MF tick every 1.5 seconds... if you don't achieve these numbers it means your rotation isn't as effecient as it could be.
Thats plenty to do, if you can reach those numbers, fix your gear, and actually research spriest dps and you're still performing badly then you can actually post some specific information and ask specific questions instead of giving such a broad pointless topic to discuss.