That route will definitely knock some miles off the trip, but not necessarily any time. It is a two lane road much of the way with a good deal of traffic. There are also a bunch of small towns to go through.
I've made the trip from Charleston to Atlanta many, many times and tried just about every cut-through there is. My personal favorite is Hwy. 178 through Pelion and North.
Take S.C. Exit 39 off of I-20. Travel through Pelion and North on Hwy. 178. About 5 miles past North, pick up Hwy. 172. Don't miss this turn, or you will have to go through Orangeburg and lose any time savings that you may have gained. Hwy. 172 will take you to I-26.
There is very little traffic on this route and only two towns to go through.
I think this route is the best compromise between maintaining interstate speeds and cutting the corner off of Columbia.
Here's a Google Map link:
http://maps.google.com/maps?f=d&hl=e...0029&z=11&om=1
Happy travels!