But as far as Superboy's tattoo: as you say healing factors of meta humans are really just human healing on a larger and grander scale and when humans heal as scar is left behind because that is the body trying IT'S BEST to get you back to what you look like before hand, skin color and all. So if you take that basic thinking and put it on a larger scale then there would be no scaring at all.
But really that all depends on how strong the actual character is. Take Superman for example: The idea behind him is that he is us but on a Paul Bunyan scale (as his current writer Grant Morrison puts it) so if we can only get away with a scar, Superman gets away with a clean face because everything about him is super.
But then we get to Superboy who is a watered down clone (he may even have less of Superman in him than before) so we don't how good his healing is because he gets KOed from some broken bones or a sword in the gut. Superman basically dismisses the possibility of a brain hemorrhage or the thought of a group of bad guys growing in size inside his brain as nothing to worry about (but the latter is more of a durability feat than a healing one but you get my point) also when he was being shocked in issue #2 the doctors said that his heart rate went up and then instantly returned back to normal like nothing happened. Also when they came in to get a blood sample from him his exposed skin healed up so fast they had no time to get a needle in. And all of this was while his powers were still at golden age levels (but the above brain thing was when his powers were stronger 5 years down the line)
this all goes back to Superboy who show that without his TK he has no where near that sort of power so he may not have that level of healing.