That information is not entirely true.
I have be able to spam a 1.5 CAST spell, get the cast bar to show interupted, which means the the server got the message and the spell was finish, but the client still showed it as casting, and this is what happens now with /stopcasting.
There needs to be a little work done on the client side to be able to distinguish from client side messaging showing interrupts, and server side messaging showing interrupt->casting.
Manly has shown this change to a nice effect in
This Post. It is not the best it could be, but it's a lot better than it was.
Now remember for true instant casts, whether it succeeds or fails the time between client->server->client is always less than the GCD, so it has no effect either way since the GCD is handled client side to begin with.
The most amount of changes I see occurring, which happening with this last patch too, is that functions that took no arguments to execute, i.e SomethingToDoSomething() will now require SomethingToDoSomething("argument"). Not a major change, just a minor annoyance.