Draftexpress has been rolling out some alternative stats, including +/-, for college players. Here is a table of prospects sorted by team defensive performance with/without the players. The positive numbers at the top of the list are worst, the negative at the bottom are best.
All the usual caveats about relying on +/-, especially unadjusted, and perhaps even moreso at the college level, BUT--Wall is in some very good company, with an extraordinary -7.4. That figure puts him among players with very good defensive reps, such as Greg Monroe, Derrick Favors, Al-farouq Aminu, and Hassan Whiteside. This part of the list is heavy on potentially game-changing big men, though there are some other PGs/combo guards in the general vicinity (including Jon Scheyer, which is itself probably a good caveat).
One interesting note is that Eric Bledsoe, Wall's primary backup, fares very poorly by this measure. (Incidentally, there are many other stat areas where Bledsoe fares poorly--I would not want the Wiz picking him, even if he slides to 35 and even if the team weren't already picking a PG.) Assuming Wall is a good defender and Bledsoe is a bad one, they may have been driving each others' stats to greater extremes. Cousins and Patterson both come out in negative (good) territory, but nowhere near Wall.