If the screen is being bleached out by bright sunshine, turning up the brightness doesn't have any meaningful effect, whether it's a Nav V, a Nav IV, a 660 or the 62" TV in your sittingroom. If anything it will make the problem worse, not better as you will lose the contrast.
Really, it only happens when the sunlight strikes the screen from roughly over your shoulder; travelling say north on bright sunny morning or afternoon. Shift your butt on the seat or alter your pose to create a bit of shadow, that is all you need to do. Nobody rides around glued to their GPS screen... Or at least they shouldn't. Just adapt and adopt to circumstances as they change; it's free and confirms your Adventure biking credentials.