My experience (could be wrong);
* Damage etc. is unaffected
* Going higher level in an ability doesn't lower AP costs. However some higher leveled spells have AP-penalties if your ability is low level, so it does negate those, but that's not really lowering cost, just removing the penalty.
* It allows you to learn more spells of that element at the same time.
I can confirm.
But people should think about that there are some really nice spells on level 13, 16 and 18 for which you have to take severe AP penalties if you only have little knowledge in an elemental school. So specializing in two schools seems to be the "best" way in my experience...
