AMS published an article few days ago that the effect was quite small - they compared speeds of Piastri and Leclerc before DRS flap opens (after which mini DRS is no longer visible) and they didn't notice a big difference in speeds up to DRS opening point. If miniDRS had a significant effect you'd be able to see it before DRS opens.
"Journo saying it was worth a tenth" seems to be that Red Bull claimed that is what McLaren gained - in this article you see mention of it https://www.auto-motor-und-sport.de/for ... en-verbot/
"Red Bull claims to have calculated that this trick is worth one to two tenths"
"The FIA initially saw no reason to act because McLaren had passed all stationary stress tests, because all wings always bend in some way and because, according to their calculations, McLaren had no significant advantage over its opponents."