2) I was going by the simcraft numbers on their website. Ret paladins technically aren't the worst, but they're close enough, and judging by the T14 heroic numbers (which are close to what a player will do in lei-shen appropriate gear), everyone could get there. also, it was just an example. maybe it's another spec, but every spec is capable of pulling 110K DPS.
3) while a boss is easier to kill with higher DPS, usually the DPS isn't the real issue, and even if it is, respeccing or rerolling shouldn't be needed. to take raidbots numbers: combat does 135K on lei shen, Assassination 160K when going by top 100 parses (all parses is 121 and 128K). but you deal with confirmation bias: many good rogues go assassination because it theoretically does higher DPS. but those rogues would just as easily kill lei shen as combat, because combat still outperforms the required DPS by around 20%.
4) maybe rogues weren't the right example, but take warlocks. Demo DPS on lei shen with top parses is 193K. Destro is 173K. if we have a player who played Destro since MoP launch and can reach 90% efficiency, but has never played demo and will only reach 80% efficiency with that because he isn't monitoring his corruption well or whatever, that player will do 156K with Destro and 154K with Demo, so that player should actually play Destro. you probably have something similar with Frost and Unholy DK. I hate the unholy playstyle and perform worse with unholy than with frost.