I agree that, one way or another, the Spurs have to get better defensively. At this point, I think the most likely way of that happening is via individual improvements and a re-commitment to team oriented defense. On the whole, I think the addition of Splitter will help this.
As far as the wings go, I'm beginning to wonder if the role of "stopper" is one that should have left with Bruce Bowen. In the last two years, the Spurs have given plenty of minutes to Ime Udoka and Keith Bogans with the idea of them being defensive role players, but have had fairly little success. If you have the right personnel, like the Spurs did with Bowen, then it's a role that can play great dividends. If you don't have the right player(s), then it can jam up your entire rotation system. I don't see that player in the Spurs pipeline or on the free agent market. If the Spurs truly want a defensive stopper good enough to lean on, they'll probably have to obtain him via trade.