Basically, any spec points that take you over the level of the spell will make it hit/effect for the maximum. Mez/stun/roots aren't affected, but healing for example will do +25% every time if your spec is artificially boosted over the level of the spell. Buffs are affected the same way. DD spells will start at +25% if the spec is artificially boosted, but the actual damage will of course be moderated by resistances, etc.
So if you have a natural 50 in your spec line, bonus spec will not help much. But if you can get up to 50 with items, you can save points for other things and still get full spec bonuses. Remember that there are extra bonuses for realm ranks as well.