I haven't been in-game for a while, but it was something like +1 speed per enchant if I recall, maybe slightly less (?). In any case it was rather unimpressive. I don't think many players will be that interested in blowing XP/SP on that enchant except possibly for class-only nuker Oly, because in most other situations +1-10 or so casting speed isn't a terribly important difference.
Maybe it will be something for healers to pay more attention to if the effects carry over to Inquisitor stance. I haven't gotten to test anything on PTS since I'm moving, and I would like to assume that enchanted passives carry over to the "transformed" state, but I don't take much for granted when it comes to NC.
I believe (perhaps I should say 'I assume') that there is no max to the enchanting. However, since the % probability of a successful enchant keeps going down, it will eventually approach -0-%. So if we get +3 cast speed for each +2 enchanting, and if the % goes down about 5% per +, then if someone were to enchant to where the success rate was about 10% (and it would be surprising if anyone got that far) then cast speed would have gone up about 27. Check my math on that.
90% fail ./. 5% decrease each + = 18 successful enchants x 1.5 cast speed increase each = 27 cast speed increase.