Yes.
Chip, Hank and I were at a contest in Elmira many years ago. Chip was not convinced about winglets for the -24 yet.
We had an off day, so testing insued.
Chip flew his -24 with an early "Nixon slip on winglet" (no wing cutting needed), I flew Hanks -24 with an early Nixon winglet that required cutting the wing and Hank flew his -20 chatting and watching from behind so he could see.

We did straight runs at different speeds to check glide as well as thermaling with Chip and I alternating who led.
Minor gains on the later winglets vs. the slip ons, both were better than no winglets.
The big difference was in, "Yank and bank rough thermals". I could fairly consistently turn inside Chip in a hot spot and get above him.

So yes, stock vs. Nixon slip on or cut the wing was better with a winglet. Later "cut the wing" Nixon winglets were even better all around.
Not sure if they were tested against other brand winglets, likely, but I don't remember.

AFAIK, Chip may still even have those slip on winglets........