There are many ways to manage development and prevent a 10-story building from being built on un-suitable locations.  Restricting the “slope” or “elevation” seems to be the worst approach for our area.  It has features that would seem to border on “the taking of land” from lots of property owners. From a safety standpoint, you need to demonstrate the potential for catastrophic consequences from building on high-slopes  to offset the property rights aspects.  You know the kind of stuff that happens on the unstable formations in the California terrain after heavy rains.
By the way, have you ever driven up Highway 176 from Tryon to Saluda? Talk about some steep slope cuts; and we only see rare occasions of rock slides in areas where there are near 90 degree slopes.  This should tell you something about the geology of the area and its stability with high degree slopes.  If there is an argument for a “slope” oriented MRPO, then the argument should be looking at slopes much higher than the 15 or 30 percent that I’ve heard offered.
Another part of a “slope” oriented rule is the need to provide exceptions.  The exceptions are based on using an “arbitrary” slope to require an in-depth engineering analysis for property owners that want to build on so-called “unsuitable” slopes.  Two points:
(1) It will be so expensive to many landowners that building will become impractical (again, sounds like a taking of property) and,
(2) An (easily contested) engineering analysis of this nature takes “specialized engineering experience” and knowledge.  Ask the engineers that originally designed the I-26 cut up Saluda Grade about what they didn’t know when they started.
A “slope –based” rule is going to be too expensive and impractical to implement.   It would put a hardship on residents/landowners that can ill afford the significant engineering costs of building on land they’ve owned for years or generations. It’s time to start looking for alternative approaches instead of trying to use fear to push through an agenda that seems directed towards aesthetics, not safety.  Speaking of aesthetics, have you ever seen Columbus or Tryon from up on top of the mountain – talk about light pollution.