We need to look at the off healing of this class. The underlying problem is naturally the fact that Arena has dampening while BG does not, and as such, whatever this self healing is, might be perfectly balanced for Arena, but it is way too insane for BG.
Sure I get it, we could be making an Arena Tier list and a BG Tier list, and I could stick to Shaman for Rated BG to exploit this difference, but I rather have we fix it.
Since 11.0.5 Enh and Ele shaman has become 1v1 gods in BGs because of the insane self healing. It is close to a reverse Disc Priest. Where disc priest is 1.0 healer and 0.75 dps, giving you 1.75 throughput, dps shaman speccs are now 1.0 dps and 0.75 healer, again giving you 1.75 throughput, outperforming the Feral self healing.
This is not good. I understand that you have painted yourself into a corner now, where you cant reduce the self healing because then the class cant compete in Arena, so you need to look at a different way to balance this.
Suggestion:
Add a Self-healing debuff modifier to BGs to motivate team play and reduce the god status of self healing classes in 1v1 fights.
No, the fix to your specific problem (at least for the Ele case), is that next week Ele shaman will become 0.75 dps and 0.75 healer giving 1.5 throughout
Healing is the only survival tool of the enhacement shaman, and in this expansion it is considerably lower than in DF, which ends up being ridiculous low when someone apply healing reductions. Not to mention the huge amount of damage we lose when we are forced to spend maelstrom on healing.
There are classes that should be reviewed with more priority than shamans (enh).
Also add that one of our main defensives with 3 min CDs can be purged or stolen, which is not the case with the warlock shields. Reducing enh shaman healing would make it as incompetent as it was throughout BFA.
Also, when has 1v1 been balanced? (not even 3v is balanced)