Background - I took up dragonscale lw to create the Netherstrike Set and keep my mana efficency up via the [Drums of Restoration] instead of being slave to mana pots (or at least as much of a slave to them).
To my dismay, the set carries 3x Blue Gem Slots coupled with compelling socket bonuses. Since I do want to retain the bonuses and not limit my options for a Meta Gem, I decided to go with 3x of the +Dam/+Spell Pen Gems. I chose this as I stay right at hit cap with gear + spec yet I am often getting partial/full resists on Raid targets.
Forward - I was recently a bit 'called out' for my 'poor' selection of gem. Which sparked a bigger debate for me:
How effective is Spell Penetration in PvE Raiding?
I have done some research and come up with little to offer in ways of support or refuse for the stat. Little seems to be known. I do know that my Nature Spells are not bolstered by CoE/CoS and that being able to penetrate some minor amounts of resistance can't be a bad thing.
What is the (if any) resistance of a Raid Mob/Boss under normal circumstances?
What is the (if any) resistance of a Raid Mob/Boss under special resistance circumstances? (Like SSC, The Eye)
Usually I'd agree with you moogle. However, don't we usually base that on the fact that the locks have COE/COS up? With an elemental shaman, wouldnt they still have to burst through that extra resistance the mob has?
Unless someone discovers that a boss has a resist rate at or above 20% over the course of a few weeks throwing lightning bolts at it, I sincerely doubt it. Very few mobs have any sort of resists at all, they are generally either immune or everything hits/resists at an expected rate. If this does turn out to be the case then a small amount of spell penetration might prove useful.
Originally Posted by CheshireCat
Eh, my nostalgia goggles aren't as good as they used to be.
Almost all the partial (and even some total I think) resists you see on raid bosses - and any mob with a level higher than you, i.e. 71+ - are due to level difference, and cannot be overcome by anything at all.
In PvE, 99% of the mobs you face have zero resistances, making spell penetration useless against them.
Molten Giants/Destroyers and Golemagg - fire resistance
Azuregos - frost resistance
Chromatic and Nether Dragonkin (UBRS, Netherstorm, Nazan in Hellfire Ramparts) - resistance to many schools
Arcane Protectors - ~150 arcane resistance
That should be about it, correct me if I missed something.
All other mobs you face have zero resistances (some are immune and you just have to switch spells scholls / raid spots, but that's another matter), having CoS/CoE will only increase the damage done by 10-13% and have no other impact.
No resistance there, nothing that spell penetration will change. Except for eating up item budget.
Unless you want to gear only for farming Heroic Ramparts, just forget about spell penetration.
Magmadar and his hounds are the last mobs that I can think about that had any resist (fire) short of immunity. In the vast majority of cases, that spell penetration does absolutely nothing. It's a pvp stat to counter resist gear, pally auras, shammy totems, and drood buffs.
The Big robots in Kara in the library have about 150 arcane resist.
To the OP, even after all this conjecture, I don't think there is any way that spell pen can be better then other gems you could have in those sockets, even if mobs do resist. I'd suggest 9 spell dmg for all your sockets.
Off to Wowhead to find a more suitable gem. >.> I think I recall not really seeing anything of interest before leaving me fixated on the Spell Pen to begin with.
Unfortunately the only stat other than spell penetration that can be found on blue/blue hybrid gems that would be of interest to a DPS caster would be mp5, which currently is never on a gem that also has spell damage (only healing). I did find a couple gems that may work for you though (all green gems, all from heroics).