I did some tests with different filler spells to check out what dps they do. Curse of the Elements was used in all tests
Searing Pain: 1450 dps
Incinerate: 1470 dps (without immolate ofc)
Shadow Bolt: 1900 dps
Drain Life: 1080 dps (with haunt + 1 stack of shadow embrace)
Dps with only CoA, Corr and UA with 0 clipping and 100% uptime on dots: 1400
My stats:
2888 spell dmg
14.77% crit
17.66% haste
54/17/0 spec
So Searing pain and Incinerate does about the same dps, but Searing pain has a shorter cast time, so its better to use in PvP. They both prevent shadow school lockout.
Shadow Bolt does most dps, but has a horrible long cast time so its difficult to use in PvP. And ofc drain life heals you.
It would be awesome if blizz gave us a real nuke spell in cata that we can use in PvP, so we could actually do something else then spam dots and fear on everyone around you. well... we can nuke someone with searing pain/drain life or whatever but it wont be very effective.