[Why are there maximums on the Appearance sliders?]

No, you can't increase the limits of the sliders in SL.

I wear a mesh body and the sliders impose limitations that restrict my mesh appearance more than the original SL shape so I was interested in this, but unfortunately there isn't a solution for this other than exploring other 'attachment' products.   Which like you said would not operate with physics like the natural SL shape or a mesh body shape. 

It saddens me that the responses to your really basic and straightforward question were not real answers but negative conjecture about you as an individual.