I was recently complimented by an inustry expert for stating the best explanation on the topic of overlining he had heard in 45 years of fly-fishing, so I figured I better post it here too. The guy who started the thread on another site was talking about using an 8wt line on a 6wt rod, since a rod that he built which the blank mfg called an 8wt cast more to his liking with a 10wt line on it.
"Keep in mind that a heavier line does not inherently fly better into the wind, a tighter loop does. To understand why, think of a golf ball versus a tennis ball. The golf ball weighs roughly 1.6 oz and the tennis ball weighs roughly 2 oz. If you teed them both up and whacked them with a Big Bertha, which would you expect to go farther? The golf ball will, due to its reduced wind resistance even though it weighs less. Casting loops work the same way.
With equal size loops, the 8wt line will be more effective at beating the wind than the 6wt, but overlining a rod generally results in bigger loops, not smaller ones. The greater weight puts more flex in the rod, which generally increases the distance between the top and bottom of the loop.
An 8 wt line will turn over a big/heavy fly better, but that's another matter all together."