Anything into Asheville would be for the sightseer tourist crowd only as the lines into Asheville, whether from North Carolina or Tennessee are both very slow, and the line up from the south through Spartanburg is out of service.
"Very slow" is a bit of an exaggeration. Both are mainline railroads in excellent condition. Just some curvy turrain. Slower than 79 for sure... But plenty of 40-50 running. Similar to the Crescent in Alabama.
I don't have a passenger train schedule for when they ran them at my fingertips, but if I recall correctly the average speed worked out to be about 30 mmph. "Plenty of 40 to 50 mph running" does not get you a 40 to 50 mph average speed. If you ahve plenty of 40 to 50 mph running, you might be able have something on the order of a 35 mph average speed end to end, but not much more.
I, however, do have a timetable from 1972 courtesy of Bill Haithcoat. The line covered 138.9 miles (Salisbury, NC to Asheville, NC) in four hours even, for an average speed of around 35 MPH. Of note, there were six non-flag stops and eight flag stops, so it might have been possible to drop a few minutes without all of those stops, depending on how Southern was with runtime calculations and padding.