Game! - Where the stick is mightier than the sword!

is there a point at which your skill proficiency doesn't affect the skill's power anymore or does the skill infinitely improve from use?

Spells have a fixed maximum damage which can't be exceeded, regardless of proficiency level. Though monster weaknesses get applied after that number is calculated, so you could exceed that maximum in that case.

Skills don't have an explicit cap on damage or otherwise, so you can continue gaining proficiency in them and reaping the benefits. However, the cost to gain proficiency levels rises exponentially.