Well, in my experience Divine Benison is exactly like Fast Cast. It reduces recast by about half of the casting time reduction and stacks with other forms of Fast Cast additively. I tested level 75-95 Yagrush for their fast cast values on the test server and didn't see a single quick cast, but it may be possible to begin casting Erase and not have the server register your change in position (thus cast while moving) as long as you're casting fast enough.
tl;dr: Divine Benison gives Fast Cast until proven otherwise.
The "Fast Cast cap" is implied from recast tests. Above 80% Fast Cast, you can't see a change in recast. So either Fast Cast is capped or the contribution of Fast Cast to recast is capped. If you wanted to test this, you have all the necessary tools.
Go WHM/RDM with Yagrush 95, 5% Fast Cast in gear (like Marduk Jubbah), and AF3+2 pants and see if all of your spells cast instantly/are interruptable. If they don't, then there is a Fast Cast floor.
50% (Traits) + 20% (Yagrush) + 10% (AF3+2 Legs) + 15% (/RDM) + 5% (other Gear) = 100% Fast Cast