You know, FFXI also takes a bit to interrupt some times ignoring Spell Interruption rate down because..and this may be a shock, the game is just designed to allow some casting to go through before interrupting. Other MMOs aren't designed that way - Heck most MMOs don't even utilize "specialized" stats beyond the basic str/dex/vit and so on.
This issue plus the "have to cast again to update position" is screwing a ton of people. If your last cast 'updates' your position inside the aoe because you couldn't interrupt it in time, then you have to cast again when out. And that's not always possible if you're not already targeting the boss and thinking about a instant cast (anything with a cast time won't have enough time to go off).

That could easily be fixed by not updating position based on last skill/cast. Just that normal 0.3s thing, no 'special pleading' update.