We've regularly used a 5mm eyepiece on our 82mm S2 giving 88x with ~0.8mm exit pupil, and find that quite comfortable and useful in good conditions. Lately out of curiosity I've tried 3.5mm (126x, 0.5mm) which is still surprisingly usable here in good enough conditions, especially across lakes where we most often want it. Eye positioning becomes finicky with so small an EP, and focusing due to lack of fine focus, yet brightness/contrast still seem adequate even in less than full sunlight. So magnification ~1.5x objective diameter still seems useful. I'd like to hear what others have found.
I haven't done resolution tests, but subjectively for what it's worth, even at 126x the daylight view feels normally detailed rather than exhibiting obviously "empty magnification", as often seems the case here when viewing planets.