We were overpowering Keepers in WotLK, and there's no lore to indicate Lei Shen was any stronger than other mogu warlords save his catching Ra-den by surprise. I'm not about to argue where he stands in comparison to Garrosh (because characters will ultimately be as strong as Blizzard needs them to be in any situation) but they're both bosses that were upheld by stolen power.
I otherwise agree with you, though I'm not sure Blizzard does. Even if the bosses themselves are all over the place, they've already come out with their intent to keep escalating each expansion's antagonist.