An excellant point.
Once Van Dorn took the "Volunteer" Arkansas and Missouri Troops to Mississippi in early 1862, you were left with a "Conscript" army. That was not the case with Texas troops particularly Cavalry, which well suited the Texans. When you took the Texans horse away and made them Infantry what happened? The second wave of "Volunteers" of Texas troops headed east were just getting to Arkansas. All you have to do is compare todays Volunteer United States army with the draft Army I was in in the 60's and 70's.
There was a marked difference in the quality of troops between Arkansas, Missouri and Texas at that time. But, if you check the Arkansas and Missouri troops east of the Mississippi they fought as well as any. It is only in the Eastern Theater that one can compare apples to apples.