Simply replace my word ''GCD'' by ''Abilities casted'' in my entire post. But it changes nothing it, was simply for a comparison purpose.
I understand that Arcane Blast is a 2 sec casting time, while NT has a base GCD of 1.5 secs. The more haste we have, the smaller the difference will be between the two. Given we are in 5.4 and the amount of gear we are running with in 5.4, it is safe to compare 3 x NT vs 1x NT + 2 AB. The difference in time per 3 abilities casted is practically irrelevant compared to the difference in effective damage gain provided by the latter. Like I previously said, 100%/400% of effective damage means that 3/4 of your damage is wasted by the multi dotting strat. So even casting 1 NT + 1 AB will be better than casting 3xNT