I found out your values for the Saturn (from your Optimal timings thread) to work perfectly in Line2X too. And also, the H. Mask/’pinching’ issue in optimal modes seems entirely fixed, now, with the new firmware.
I did actually try Mega Drive’s optim. 256p in 4:3, but the image is visibly stretched (too wide), to an extent it looks indeed wrong. Nothing I can seem able to rectify via monitor’s settings, unfortunately.