Now this does not change equipped signets into a fast casting skill but instead causes the skill to use the fast casting attribute to determine its effectiveness. So would a build focusing on the use of this enchantment want to use a fast casting 40/40 set? Or would the gear give no benefit unless tied to the original attribute of the signets?
I don't think the 40/40 applies to non-Spells anyways, so I think this is irrelevant. That said, the Signets use your Fast Casting attribute, but the attribute of the skills does not change.
When my mesmer runs Symbolic Celerity she uses Rajazan's Fervor and an Equine Aegis with +45 health while enchanted. +20% longer enchants are there and the armor bonus is higher than what a staff could provide.