My game tells me what damage my spell is doing; they appear transiently across the bottom of the advenuring screen. When killing the big dragon, I distinctly remember reading the damage my Poison Clouds did. However, there are limits. If you have a spell that hits multiple monsters, you won't get a report on every one.

There are definitely benefits from adding to some skills beyond grandmastery, but it depends on the skill and the character. This is a fairly complex topic. For example, IIRC the damage an assassin does with a throwing weapon is not only tied to his skill levels in Thrown Weapon, but he also get an extra attack for every 3 levels above the 10 he needs for GM. But I don't belive that particular benefit applies to any other character. The interplay of weapon skill and armsmastery has received quite a lot of attention in the past - you could try searching (Ctrl F) past pages for it.