Gotta agree with SearchEngineZ. Over the last 18 months, I have been slowly improving my ads to boost CTR. As a result, there has been a significant drop in my cost-per-click (and thus my overall cost). My average CTR went from 3.5 to 6.8 (ish), and I've seen about about a 50% reduction in CPC. This is massive for my business (and for the clients that I manage -- some on a limited budget)

That said, the impact between a 5.5 and 4.5 may be massive to your CPC, or insignificant - it depends on what kind of traffic you're talking about. If the difference is due to a handful of clicks vs thousands of clicks, then you're comparing apples to oranges.