Lightroom doesn’t let that slider go past ±100 directly. If you must stack more correction in Lightroom, export and re-import, but use a lossless or high-bit format such as TIFF, DNG, or possibly PSD rather than JPEG to avoid cumulative quality loss.

That said, the bigger issue is that this usually isn’t the right correction. Lightroom’s Manual Distortion slider is meant for lens distortion like barrel or pincushion distortion from a physical lens. A stitched panorama creates a different kind of geometric distortion from the chosen panorama projection.

So if a panorama still looks wrong at +100, the better fix is to re-stitch it using a different projection—especially perspective projection—or use more advanced panorama software that offers projection choices and perspective/control-point corrections. For very wide fields of view, some bending may be unavoidable because not every wide scene can be mapped flat without distortion.