As much as I would like to claim this FKT, I couldn't do so without doing some further research. As it turns out, Max King put in a blazing round trip time of 2:10:13 on August 9, 2015. There doesn't seem to be much interest in round trip times as opposed to ascents, but I'm more infatuated with former. That's probably because I'm a slow climber, and because my main goal in wilderness travel revolves around getting home reasonably unscathed and eating food. At any rate, the following are the fastest times I could find with GPS data on the South Sister round trip route.
1. Max King - 2:10:13 - 8/9/15
2. David Mitchell (me) - 2:22:06 - 8/15/16
3. Mitch Thompson - 2:34:XX - 8/6/16
4. Tyler Green - 2:41:13 - 7/23/16
5. Max King and Ryan Bak - 2:41:XX - 10/XX/11
Rick Floyd also put in an impressive effort on 7/19/15, but didn't seem to tag the summit.
There might be some other quick times that I haven't located. I'll work on putting together list of female times, as that interests me as well.
And for the ladies:
1. Sarah Keyes - 3:42:01 - 9/22/15 (Likely fastest ascent ~ 2:10)
2. Alexis Gibbons - 3:48:23 - 7/17/16
3. Taryn Hand - 4:07:10 - 10/17/12 (Ascent 2:12:39)
4. Amy Sproston - 4:08:32 - 7/24/16
5. Corie Townsend - 4:19:51 8/15/15
6. Rhonda Sundermeier - 4:23:14 - 8/4/13
These are the fastest times I could find while sorting through GPS data on Strava. There doesn't seem to be much info elsewhere on the internet. Other, and faster, times may exist. The route and data is a bit convoluted, however. Some choose to start their run at the Devil's Lake TH, some at the highway, and some at the permit box 100 feet down from the highway. Since Max King ripped this route apart from Devil's Lake, I think all future attempts should start/finish from the permit box at that trailhead. Many run the trail around the summit crater to gain the highest point, and many run, if frozen and runnable, across the lake at the top to reach the summit. The lake run, in the right conditions, subtracts mileage and is faster. I think the summit route choice is a matter of personal preference and comfort.
It's a great route, and I would love to see more efforts put into round trip attempts rather than simply ascents. There is no better feeling than flying down a volcano at mach 40 while smiling at folks grunting their way up this beast.