It doesn't make sense that this would happen anyway. MP is taken out when casting concludes, so if anything you'd want it swapped in just before you finish casting, not just after you start casting.
It still probably doesn't work that way, but it might be worth another test to check. EDIT: Not on anything where you'd want MAB or Cure Potency, obviously, but on something that takes forever to cast and is expensive. (In other words, it'd be useful for tele-whoring, and possibly the Boost spells we seem to be getting depending on their cast time.)
I had always assumed it procs when you finish casting as well. It's totally logical, based on when other stats are calculated. I see no reason to test it, by process of elimination we've established that it must happen at the time you finish casting. Aside from PreCast, there's no other time it could be calculated.
As far as usefulness, I am still totally in love with CMP and use as much as I can. However, other stats such as cure potency take precedence. For example, my default MidCast swap for WHM loads me up primarily with haste & CMP gear with a dash of enmity down, giving CMP+28 for a 53% activation rate. I use this set for casting spells like Haste, Protect/Shell, Raise, etc. It's definitely a huge factor in my longevity as a WHM. I use a similar set for Regens, swapping Goliard Saio for Cleric's Briault of course. Cures are farther from this than most sets, in order to obtain a bunch of Cure Potency. My Cure sets have CMP+16 for a 41% activation rate (Selenian Cap, Magnetic Earring, Austerity Belt).