I have a Lowrance i-way 500 that is obsolete BUT the brightness of the screen on high is bright enough to see clearly on my motorcycle in the bright sun.
I bought a Gamine nu-vi 775t and used it through the winter in the car in Phoenix and did not have trouble seeing it.
I came back to Indiana and put it on my MC and it is not bright enough to see in the sunlight. I have it on 100% brightness. I even purchased a sunshade that velcros around the perimeter and it still is hard to see in the bright day.
I noticed some of my friends with the Garmin Zomo that is designed for motorcycles and a couple of them have sun shades on their Zumo's. Most of my biking crowd ride the newer Gold Wings with the built in GPS from Honda and there is a molded sun shield built into the dash of these bike's.
DO ANY OF YOU RIDE MOTOR CYCLES, THAT DOESN'T HAVE A BUILT IN GPS, THAT YOU CAN RECCOMEND A GARMIN WITH A BRIGHT ENOUGH DISPLAY TO COMBAT THE SUNLIGHT? I would still use my old Lowrance if I could get up dates for it.
I can't help you with the new stuff but my old Garmin Etrex works fine with no shade.