SAR makes sense if you see him replacing Nazr in the starting lineup. Defensively, Duncan would have to nut up and guard the opposing 5. In general, I think SAR's offensive game would complement Duncan's well. Defensively you would lose some shot blocking and physical presence up front. SAR's body isn't that bigger than the average 3 in the league.
SAR isn't going to get a new contract starting anywhere near his 2004-05 salary.
Honestly, I think the Spurs need more muscle up front, not less. Rahim's offensive skills would be a welcome sight after Nazr and Rasho, but this team is built on defense and Rahim has never struck me as that much of a banger in the paint.