As with all things soaring - it depends.

In an attempt to summarise / save some reading. My understanding is as
follows:

You can take it that a correctly set up winglet affects performance
equivalent to adding approximately 50%-70% of the winglet span to the
wingspan. Resulting in higher aspect ratio and lower induced drag.

Problem is that as speed increases the form drag becomes dominant over
induced drag. So - only for a specific speed and AoA range do winglets
reduce drag (sometimes significantly). Get your design wrong and the
performance is actually easily impaired rather than improved.

Either way you get what you paid for. Some are set up for better
cruise,some are set up to improve climb performance, some are set up to
tame poor handling at low reynolds numbers, some are just tacked on
without any clear idea...

Theoretically the winglet can be set up to generate lift with a forward
component to the vector extracting waste energy from the tip vortex -
improving glide efficiency. Doing this well involves some complex
geometry and arcane design which I don't claim to understand.

On many gliders adding a winglet improves the handling, and so
indirectly improves achieved performance.

Bruce
--
Bruce Greeff
T59D #1771 & Std Cirrus #57