Skill mastery is an ability that tries to define a role for the rogue, but in my opinion it went quite too far and damaged the game. Having both +3 and minimum roll of 10 makes for a bored rogue player (i know that mine was at least) and overshadows the other PCs (why would i invest in skills if no matter what the rogue is better?). Add to this the knack skill at lvl 2 and the DM starts to be in serious problems, if he wants to provide a really challenging skill test, then he must set a DC impossible for the rest of the group.
As such i suggest the following change:
1) Remove knack at lvl 2 (see below).
2) Skill mastery allows you x times a day (3, 4 or something like that) to either raise a skill to +3 OR get a minimum of 8 on your roll. At lvl 2 you also get the chance to grant yourself advantage on a roll, and maybe additional uses/day.
This way the rogue would still be a master of skills, since he has an answer to any situation:
1) Routine check that you don't want to fail. Get 8.
2) Check in something you are untrained. Flat +3.
3) Difficult check in something you are good at. Get advantage.
At the same time you are not limiting the other PCs. If my paladin has diplomacy +6 then i'm the one that will negotiate with that guy, cause at best you have a +3. You don't have +3, advantage, and no less than 10 on the roll at the same time.
Also, it would make sense for the rogue to invest in secondary ability scores. Having a + 2 instead of -1 means autosuccess on easy checks and makes your advantage more useful.
Just throwing an idea, what do you think?