I think OKC would be better off targeting a new PF to play alongside Ibaka at C instead of going after a center. To target quickness and athleticism. That is where their advantage is against teams like San Antonio. Getting bigger and slower will only make them more vulnerable defensively to offenses like the Spurs.
Ibaka has improved a lot over the last few years. He is a much more disciplined defender, a vastly improved man-to-man defender and has improved physique to battle bigger centers. He wasn't ready to play center earlier on in his career but he is now. Oklahoma should strongly consider moving Ibaka to C full time.
It'll be much easier for them to find a good PF than to find a talented C to put alongside Ibaka.