I make the trip down sometime from the middle to the end every March each year and have the Gap and surounding roads to myself. Day time temps are usually in the 50's and I've found the roads clean. The trick is to not start out to early and make it back to the hotel before dark. A couple times I've found it in the 20's on the Cherohala if I started out to early, but by late morning it's in the mid 40's and 50's in the afternoon. The Cherohala is the coldest spot. I know it's a little cold, but it's so much fun riding the area with NO traffic. FJR has the right idea.
It's crazy in the summer, way too crowded. This summer guys were tagging rev limiters at 4am, you had to sleep with earplugs. Some local dude on a Muzzy motored ZX9 was awesome. I was on my ST1100, but still fun. I've taked a day off during the week and ridden my CBR900 by myself.