You'll be guaranteed to receive an experience for all quest. Without reduction of gray quests.
Install MOD:
1. Extract all files in folder (X:\XXX\Witcher Install folder\content\patch0\bundles).
2. In any case do backup files (patch.bundle in folder X:\XXX\Witcher Install folder\content\patch0\bundles). Copy file in somewhere.
3. Run .bat file (like eXPMod3.bat). Done.
Uninstall.
Run uninstall.bat (X:\XXX\Witcher Install folder\content\patch0\bundles).
Enjoy.