Here are the maths I made for Selene if anyone is interested
With my gear I have 424 Spell speed, which give me a base GCD of 2.42, with Selene it goes down to 2.30.
That's a 5% speed increase;
With 2.42 GCD over 30seconds I would have been able to use 12.39 spells
With 2.3 GCD over 30 seconds I am able to use 13.04
That is not even a full extra spell so take it as you want but I'd rather have the heal potency increase from Eos (Physicks go from 890 to 1090)
Also I am not wasting a swiftcast + 500 MP just to min/max a proc that will give barely 1 more attack