I think +2 is fine but if you are going to stack it then you can't have it be +2 on a spell that is already getting spell focus, greater spell focus or both. Therefore I would say it only adds +1 to a spell that already has spell focus or greater spell focus and +2 otherwise. This is still beneficial because there is no way you are going to have spell focus and greater spell focus on most spells in your domain so you will get a +2 to most things anyway. Just to compare ability focus give +2 to only one of your abilities such as stunning fist. This is much better then that and has a potential to give a +2 to a 9th level spell save which is pretty powerful.