I think the issue isn't sunlight penetrating the screen and hitting the gauges, it is the sun reflecting off of the underside of the screen onto the gauges in a focused point, when the sun comes in from behind. Think of parking your bike with the ass end facing west on a hot afternoon.
In that case, a dark or mirror screen won't save you.
* Last updated by: jtemple on 7/30/2013 @ 7:12 PM *