Yes it will interrupt spells but counting on procs not a good idea. If it is that important there are plenty of other ways so interrupt casting. one pally/cleric/chanter can easily use spells and all other classes can use push to stop gate and cheal.
probably just up to 55 like usual...but its still very nice for agro...if youre a tank and wondering what to put in your weapons, this is probably one of your best choices. (although warriors get an anger iii aug which would probably be better agro for them)