Good to know about the 20/4. Every now and then I get sent a free autoflower seed, but I usually give it away or end up not ever getting around to germinating it. My buddy ran one on 12/12 with his other normal females. Regardless of being under 1200 watts of HPS, the thing didn't yield squat. I also am kind of weary of running an HPS 20 hrs a day, but then again, I'm in a pretty warm climate. I'm sure your ventilation is set up efficiently so you shouldn't have to worry, at least from what I've seen.
So in my opinion, as long as you can put it under the 20/4 light cycle and not a stupid 12/12 (my buddy is an idiot LOL) I'm sure they will be freaking awesome! I'd rather germ a bag seed and hunt for hermies than put an auto flower under 12/12. It just looked like it grew under a few CFLs haha. I will say the quality/smell/taste was some of the purest citrus flavor I have ever smelt or tasted from a strain, and it was an autoflower.
My other buddy did his auto flower the right way for his first grow and yielded 1.5 dry ounces from one plant with a 250 watt HPS. It was one solid cola! That strain was La Diva. It was also a little stronger smell wise than most other 12/12 buds I've smelt. That's probably my favorite thing about all the auto's I've seen, is the terpenes/hydroterpenes (smell/taste).
Good luck to you! I'm excited to see what they look like throughout their lives. Is the GCrack a GC autoflower? I think you said you were running a GC.
Not trying to hijack your thread, but if you want to check out a few pics of some of my 250 watt cherry pie x random strain coco grow. I'm getting close to harvest time. Chopped a few already.
http://rollitup.org/t/is-67-rh-too-high-to-dry.864483/