it's all about draft
Assuming we're talking about fraction rigs here, since masthead rigs are different: a headsail other than a lapper (105-110%) will reduce the boat's ability to point. Although a genoa may help you go faster in light air, you will also be going lower, which means that you haven't improved your VMG (velocity made good)upwind. Adding additional headsail area will not only overpower the boat, it will also unbalance the rig AND you'll have a situation where the genoa is gassing the main. This is especially true if the shrouds are outboard, as they are on most Hunters. Hence, the best way to improve performance is to concentrate on sail shape rather than sail area. A kevlar sail, for example will hold better shape, meaning that the draft will be more forward than on a stretched-out made-in-Hongkong piece of $#!% such as the OEM sails that come with most lower-end production boats. When a sail stretches, the draft moves back, which means more healing and less pointing upwind. (Have I mentioned the fact that I've had to recut my OEM UK main THREE TIMES in its first two years of service?) In terms of downwind performance, Phil's right about fuller being better because, duh, the draft moves forward. What this means is to get a drifter, genniker or spinnaker. (And/or a whisker pole.} The basic rule is this: to increase performance on a masthead-rigged boat, buy bigger headsails; to increase performance on a fraction-rigged boat, buy better headsails. And a better main. And a whisker pole.