If the Genny is significantly larger than the main, are you better to take the main out to windward (within reason) to get more drive from the larger sail area?????
When you're running downwind, wing and wing, the boat is not being driven by a laminar flow of wind over the sails. It's simply being pushed downwind by the "weight" of the wind on the jib and mainsail. If the main is eased too far forward, then the excess wind spills over all sides of the sail. If you trim the main a bit aft, then the excess wind will be funneled toward the jib, and it'll pour off the luff of the mainsail onto the jib, and it will add slightly to the total "weight" of wind on the jib. The available wind drives the boat when it strikes the mainsail, and then it provides a little more drive when it pours onto the surface of the jib.
When the wind is pouring off the main into the jib, sometimes it can create turbulence and start spilling over the leech of the jib. When that happens, the leech will start to curl, and shortly afterwards, the jib (if it's not poled out) will collapse. You can greatly reduce the likelihood of that happening by either setting a whisker pole on the jib, or, if you don't have a pole or are too close to a turning mark to be able to set and take down the pole, by trimming the jib a little further aft, to prevent the wind from spilling over the leech of the jib.
The general idea is to use the sails to "capture" as much wind force as you can, and don't waste any of it by letting it spill out of the sails before you have wrung all the driving force out of it.
This works on a boat with a masthead rig, which has a big jib and smaller mainsail, but not on a fractional rig, which has a big mainsail and smaller jib.