How raid viable is Ele?
A unique challenge for Ele is spell resistance. Neither Curse of Shadows nor Curse of the Elements reduce Nature resistance. I don't know precisely how much resistance bosses tend to have, but I think it's around 50-70.
In this video, I have 70 spell pen (in late AQ/early Naxx gear). As you can see in the combat log, there are still partial resists. This is because bonus spell resistance from being a higher level cannot be penetrated. It's believed to be 8 resist per level, so a boss mob would have 24 resist to all (even Holy) that cannot be reduced by gear or curses.
I think 70 spell pen is probably excessive for this reason. How much do you think is needed to fully penetrate the typical raid boss?
I made 133 spell penetration with naxx gear.
It's an interesting question. The unique challenge is the relatively high Nature Resist bosses seem to carry post BWL, relegating Ele Shaman to excessively sub par. Additionally, as the OP has stated, neither CoE nor CoS reduces boss Nature Resistance specifically. That paired with an Ele Shaman's problematic mana management and lack of backup ranged weapon (aka Wand) sees Ele Shaman at a lower impact threshold than the vast majority of their ranged counter parts -including even hybrid ranged.
This video in the OP poses the question as to how much spell pen would be required to full hit the boss. I think it wouldn't matter considering the amount of NR inherent to late game bosses, paucity of spell pen, and sacrifice obtaining that spell pen would cost in relation to spell damage. I'm not sure Ele Shaman will ever be in a state that they can engage with the rest of the ranged DPS club.
Please, prove me wrong. I would welcome the spec diversity in high-end raiding.
@Egregious I think spell pen is not as rare as that. I doubt many bosses have more than 100 NR after you take into account the ~24 from their level that can't be penetrated. As @Cromas said, you can build sets with a ton of spell pen if necessary.
As for the mana usage, I think that's also not really a problem. With aggressive consumable usage and downranking to Lightning Bolt Rank 4 (as I did in the video) you can maintain your mana indefinitely. LB Rank 4 gets the full coefficient from +dmg in vanilla; upranking is paying much more mana just for the higher base damage. Still, you can see a precipitous drop in my DPS when I switch from max rank to rank 4 in the vid. But factor in raid buffs, world buffs, and compression time and I don't think mana is a major concern except possibly on progression.