To reproduce: 
1. Add a RadRibbonBar and a RadButtonElement in one of the groups.
2. Set the RadButtonElement.UseCompatibleTextRendering property is set to false.
3. Set the RadButtonElement.Enabled property to false.

You will notice that the text is hardly read compared to version Q2 2014.

Workaround: Currently, the possible solution is to set the RadButtonElement.UseDefaultDisabledPaint property to false and subscribe to the EnabledChanged event where you can modify the ForeColor in order to make it gray when the button is disabled and apply a gray scaled image.