I've played with people who shot a 26, I've shot a 27 and I know a couple people who've shot a 54 (18 holes). I have no reason to believe that anyone-that I've played with- is cheating. It may be unbelievable when your 30 gets trumped by a 27, but that's golf and I can insure you, some of the great players out there, really are that good.
If you scan through some of the best players' score history, you'll notice that they don't consistently shoot 27's. Infact, far from.
I've had rounds where my average distance from the pin is 5 feet, so if I can do it, heck anyone can! It is possible.
BTW, I'm not saying cheating doesn't exist, it just isn't clearly evident as you explain.