i don't think your gemming is wrong, per se, though there are a few changes i would make. i'd put the Icewalker enchant (12 crit/hit) on your boots.
until you get a helm that allows a meta, you'll be better off just gemming straight spellpower in your chest. until they're required for the meta, blue gems are less valuable than straight SP, except in single blue sockets with an +8SP bonus.
i'd recommend taking points from either Improved Vampiric Embrace or Focused Mind and putting them into Improved Shadowform. i love VE and use it tons, so i don't have any points in Focused Mind. >.> there are debates on its utility, but some people love receiving
any heals. additionally, i'd pick up Inner Focus, but there's also some debate about its utility. Imp. Shadowform is helpful when taking raidwide damage, which is apparently prevalent in Ulduar.
i think this other shadowpriest is... ahem. willfully incorrect on a number of levels. your hit is just fine for raiding, and while there's spirit on the gear (healy stat), it is just
on most cloth gear, and an upgrade is an upgrade. you are perfectly correct in trying to maintain your hit while gaining crit -- you do want a minimum level of haste (the 'soft cap' of 300-400 to get two full Mind Flays in a MB cooldown), but it needs to be raised without sacrificing gobs of SP, hit or crit. he might also be confusing hit with haste.
i've compiled a bunch of tips and sources in
this thread, but you'll probably want to check the
Best Raiding Gear threads and the
stickied spellcasting thread.
firstly, make sure you're not clipping your dots. i'ma copy-pasta from that thread above, but hey, i wrote it. :p The last tick of VT is also the last second of the effect, so make sure you're casting it with about 1.3 seconds left on the dot, with adjustments for your haste. You want it to land just as the previous one drops, but it's better to cast late than early; you'll overwrite significant damage if you recast it too soon. If you happen to have a gap of 1.5 sec/1 GCD (not long enough for MF2, and so long that not casting makes us a bit twitchy), SW:D can be a viable option; dropping it from the rotation may result in better dps, simply because it's challenging to neatly fill 1.5 sec gaps with any kind of consistency. there are mods which help you with the timing of casts, but i don't personally use them; you do, however, need to refer to a mod to determine your spell uptime while at a dummy -- you want to at least hit 90%, since you'll likely maintain a lower uptime while raiding. your opening rotation is fine, though i'd adjust it to VT, MB, DP, MF2, SW:P just to get the MB juggling up that much sooner; priority after that is just VT > MB/DP > MF, but since you're hitting 3k, you likely have that worked out.
just ignore that other dude -- might just be sour grapes since you have a number of best in slot gear pieces.
