I went from a 15/39 to a 15/41 which is a slightly smaller change than going to a 14/39.
I prefer sticking with the 15T up front because the smaller radius of a 14T can potentially accelerate chain wear and wear on the slider on top of the swing arm. Opinions are divided on this issue, but I tend to go with the conservative side to minimize the potential for wear.
Either way, I don't think you'll see a significant drop in fuel economy or top speed. This has been discussed well in other threads so I'll just go with the short version: If the stock gearing doesn't allow a specific individual bike to reach it's peak hp rpm in top gear, going to a lower gearing may actually increase top speed. It depends on the state of tune for that particular bike. In reality top speed will probably remain very nearly the same. Driveability at lower speeds will definitely improve as many folks have attested to.