What I think speaks for the 6637 haste threshold is that you'll get right over the cap for two of your dots meaning that even though you shouldn't drop them and thus do not benefit from the extra ticks that often you'll get a longer duration on them and waste less globals on refreshing them.
I really can't say if SimCraft is right or wrong because I haven't spent any effort in optimizing its models with action lists etc. Asking me will give you the common sense answer.
I first went for 10% haste -> mastery until I got the Elegon trinket where I switched to 6637 haste -> mastery with full mastery gems. I've been constantly ranking throughout this tier as well as playing in a world top 30 guild. Does this mean I know for sure that my gear is optimized for each encounter? Hell no.