Really good, and here's my proof.
Player A averages 36 MPG, 19PPG, 5 APG, 4 RPG, and 1.5 SPG on 40%/32%/82% splits. He signed a contract for 4 years/48 million dollar contract.
Thomas averages 26 MPG, 16PPG, 4 APG, 2 RPG, and 1 SPG on 42%/39%/89% splits. He just signed a 4 years/27 million dollar contract.
Player A is Kemba Walker. Dig further, and you find that in their contract years (2013-2014), the numbers are even closer. Walker was at 18, 6, 4, and 1.2 on 38%/33%/84% in 36 MPG. Thomas was 20, 6, 3, 1.2 on 45%/35%/85% in 34 MPG. For some reason, Thomas couldn't get anywhere close to the 12 million a year that Walker got, and had to settle for a contract that averaged about 6.7 million a year, almost half of Walker.
Advanced stats say the same things. In fact, they are both defensive liabilities according to the DRPM stat: Walker at -1.68 and Thomas at -1.9. However, Thomas has a better ORPM stat than Walker: Walker at 2.82 and Thomas at 3.25 (Thomas is the tenth best in the NBA at the ORPM stat).
The WAR stat indicates the estimated number of wins a particular player adds to his team: Walker is at 3.15 and Thomas is at 2.77.
If you argue, "But more of the offense falls on Walker's shoulders, and therefore his production won't look as good." Actually, the usage stats say that Thomas has a much higher responsibility for his team's offense. His usage rate is 35% to Walker's 25%.
Oh, and Thomas also wins the PER at 20.06 to Walker's 18.73.
I think you could make the argument that Thomas is a better player than Walker from these stats, although I'm not convinced of that myself. If Kemba Walker's contract sets the level for players of his production (and it approximately does), then we got a little more production than that for almost half the price.
That's a player that's worth a late first round pick.