Not Summoner but it can be useful for you Dervy.

At 644 Spell Speed.

Fire 1 - base 2.5 --> 2.39 -- 0.11 secs
Fire 4 - base 3.0 --> 2.87 -- 0.13 secs
Flare - base 4.0 --> 3.82 -- 0.18 secs

You definitely gain more from Spell Speed with longer recasts.

If I recall 26.5 Speed = 0.01 GCD for a 2.5 seconds spell.