I have been using a Kobo Mini with XCSoar for the last two years, and have never had a wash out problem. My wife also uses the same mini as a e-reader at the beach. She sits in direct sun light by the hour, and has not reported any issue.

I find the "ghosting" to be a non-issue in flight. After years of using a PDA I'm just happy I can see the numbers in all flight conditions. And, the entire system was great value for money, less than $100 (in a 40 year old glider).

Gary