It shouldn't matter what weight the rod is as far as casting distance is concerned. Sure wind may play a role. But a well balanced 5 wt will cast further than an underweighted 8 wt. The rod's action (fast vs. slow/moderate) also plays a key role, particularly in the wind. The key to distance casting is technique not power. Let the rod do the work, and master the double haul.
My max distance is the same for my 5 wt trout rods and 8 wt salmon rods, as long as the action on either rods are similar....about 90 feet to the backing under ideal conditions (e.g. In a park). In the salt without a stripping basket my distance drops to 60 to 70 feet as the surface tension of the water on my line will stop the line from shooting out. Like flytech, if I try to power the cast, it usually fails!