There's nothing definitive on this subject anyplace and with the latest set of changes to edges, Devious (and point extras to play around with) is a pipe dream mostly, so this is very important to know: Is there a bonus to Thug skills if the thug is going full path? Earclap and friends are marginal at best by themselves, hence the ask.

I believe the answer to your question is no. I can't think of any thief skills or other game mechanics that give a character an advantage purely for having studied the whole path. You get the benefit if having the skills, but it isn't as if a switch is flipped that instantly makes you better when you take that last thug skill.

As an side, there are dozens of characters with enough edge points to take DV, and specifically the average character level 42-51 could as well. Few characters are likely to be able to buy DV and a party bag full of other desirable edges, but I guess that's the point.

EDIT: typo.