My Discus CS had factory fitted winglets.

However the "Cut line" between wing & winglet wasn't as per the details
in the Schempp modification. (I assume the factory use a jig)

This lead me to have doubts as to the correct AoA of the winglet, so I
stopped using them (After about 5 years!)

In general use I found there to be no difference with or without, so I
continued to fly without them and didn't take the hit on the increased
handicap.

I never found an effective way of measuring what the toe in/out of the
winglet was, or figures for what it should be.

In summary, correctly fitted winglets may make an improvement but it's
easier to make things worse than to make things better

PF