Hey, Jim - Welcome to the message board!
With or without the cables, "5.11 something" would be vertical (90-degree slope), and Half Dome is nowhere near vertical (a ~50-degree slope at one brief steepest point). The biggest danger, in my opinion, besides overcrowding (or actually, because of it) is the fact that the long-time "cow path" on the granite between the two cables has been worn so smooth that now it's slippery even when dry.
Unless I totally misunderstood your point, even if horses were allowed on the trails in the area (other than stock animals), I can't see that the new permit situation would have any effect on them at all, since horses won't be doing Half Dome, since the permits are only for the sub-dome and the cables, which horses clearly are incapable of doing. (Can you just picture a horse trying to do the cables??? That would be just about like a salmon trying to swim up Yosemite Falls...!).
The Half Dome round trip from Happy Isles is approximately 16+ miles. The elevation gain from Happy Isles to the summit is between 4,800-4,900 feet -- a mile is 5,280 feet.
CaT
