While bringing up the spec question regarding invocation may be a valid one, in future please work out the actual impact of picking up invocation using real data set if you intend to advance this talent as a must have on Alysrazor.
I'm investigated top 10 mages on WoL and don't understand why nobody takes "Invocation" talent for Alysrazor encounter?
Pure +10% damage on phases is 5 mil, maybe a way more, it's very roughly number. So, overall damage for 600 seconds length fight is ~9.000 dps or more.
You can calculate the exact numbers.