@Alien, if you don't have time to wait for the subway train going direct the South Station (rather than university town south), you can get a DiDi car at Chun Rong station that will take you to the South Station in less than 10 minutes and costs 15rmb max.

Went out all the way by subway yesterday evening to South Train Station to see just how long it would take. If I'd taken the earliest morning direct subway train and if it took the same amount of time, I would arrive at 7:32, leaving me 21 minutes to get on the train at 7:53. Was told at the station that one needed to arrive for trains 20 minutes before departure. Perhaps that could be ignored, but given that the ticket gates usually close about 5 minutes before train departure, that's cutting it pretty damn close - as somebody wrote above, the place really is huge. A staff person at the station suggested I might take a room in a local hotel in the area the night before in order to make the train in the morning.
Station is fine, trains are fine, but it seems coordination between subway schedule and early-morning train schedule is not quite as good as it should be - and I note the bus schedule mentioned elsewhere puts the first bus leaving Kunming (I mean the real Kunming, not Chenggong etc.) at 7AM, which is way too late.
Was also told by same official that if I missed the train I could use my ticket for the next train, which is quite a bit later - but then aren't all the seats reserved ones, and most/all reserved in advance?

You'd just have a standing seat ticket. You can take any open seat in your class. But that means for some segments you might have a seat, and some not. I was in this situation for the Wuhan beijing line. I'd say I ended up sitting for the first half, then standing mostly for second, so like 2 hours plus standing total. One option is if there is a dining car, sit and order a meal, and stay seated there after, but the dining car fills fast and pretty limited. I'd suggest a taxi early morning, maybe 100rmb, but will be super fast as zero traffic. Or take the first subway to the south bus station and grab a taxi from there, but that's a bit more risky. I would definitely show up at least 15 min prior for G trains. Some gates are airport style, meaning they'll close prior and just not let you pass, even if you could run it to train in time. Granted I say this from experience with various other g trains in other lines, haven't taken the kunming line yet.

@Alex: your Wuhan-Beijing trip - was that a high-speed rail train? I had thought that standing wasn't allowed on such trains.

It was highspeed, reached 200 plus kmh at some lengths. Can't remember if it was sold as D or G train, as that line was alternating. I did buy the ticket as standing, so wasn't bumped into it by missing a train. Also, this was two years ago, so, policies maybe not the same anymore.

Thanks, Alex.
Note that 1st train to Guangzhou leaves at 7:13 - impossible to make by public transportation, unless, perhaps, you happen to live in Chenggong.

When we first had buses to the airport, the first flight and first bus didn't work out either. Now it does. Given some time, theyll have a really early bus to the the train station. Have to wait until then. Don't think the subway will ever adjust, as I've rarely seen a first subway before 5:40am in any city in China.

One would expect central planning to avoid this kind of problem.

A subway at 5:40AM would do it.