Drop Improved Spirit Tap. When it procs, you gain a total of what, 14 Spellpower? Wow, grats. If it's for mana, get 2/2 Veiled Shadows for both increased DPS and increased mana (3 minute cooldown is HUGE), and learn when it's okay to use Dispersion.
Keep Improved VE, it's an underrated utility that should be loved by your hopefully competent raid leader (assuming he knows which group to put you in), and it's what keeps you up during some of that hard blasting incoming AoE. Raid Damage is everywhere, it's not only going to drop your health bar, but also your cast bar and to keep that DPS moving, you want pushback resistance. Don't even THINK about dropping Improved Shadow Form.
Dropping Spirit Tap altogether slows your solo progress by about 5% (big deal... :
) and lets you pick up Shadow Affinity to Mind Sear the hell out of some of those pulls faster, without having to worry about
ever pulling aggro.
Edit: This is the progression raiding
Cookie Cutter Shadow Spec for max dps.