When using Distortion correction with lenses like 8 mm, even with slider pushed to +100 is not enough for me to do what I need. I think that Distortion effect could be a lot stronger even though necessary amount of cropping would be higher (I am counting with that already while taking picture)
What lens and camera model and are you using raw files with Lens profile Distortion correction or JPEG files with Manual Distortion control? I tested my Canon 8-15mm fisheye lens on both full-frame and crop sensor cameras and Lens Profile Distortion set to 100 provides near perfect rectilinear correction at 8 mm focal length with raw files. The Manual Distortion control is not designed for correcting fisheye lens distortion.