Scenario: replace Splitter with Howard on the Spurs. Assume identical personalities. I watched Howard a lot in Orlando. Unfortunately, I have to reduce his abilities because he's not as quick as he used to be, but here is what I think happens:

Defense: not a lot of change. Howard is not the positional defender Splitter is, but he's got the elevation and timing to make up for most of it. He also gives the Spurs two strong guys for post defense. Defensive efficiency probably stays about the same, the Spurs would just do it a little differently.

Rebounding: Howard is ten times the rebounder Splitter is. Huge upgrade at this position, especially against teams that work hard for offensive rebounds. However, the Spurs are a good rebounding team in general, so it wouldn't show up as much as you might think in team stats.

Offense: Howard would definitely post better offensive numbers than Splitter. He's much better working in the post and finishes much better. However, Splitter has the vision, court sense, and passing of a point guard whereas Howard is an average center in this regard. Howard finishes at the rim much better than Splitter. However, his screens are terrible. Elbows and knees flailing everywhere. He doesn't get called too often because he's a big name, but I'll still take Splitter's much cleaner screens and exceptional re-screens. Given how many screens the Spurs set on and off the ball, this is a bigger advantage for Splitter than you think.

On the whole, I think you would see a small reduction in team offensive efficiency in spite of Howard's better individual stats. The Spurs already have Leonard, Parker, Duncan, and Diaw as high efficiency iso players who can score a lot of points, so adding another, while nice, wouldn't really be a game changer. What you lose would be Splitter's passing and screening, which is key for the Spurs. A lot of those wide open layups and threes for the Spurs bevy of role players go away without Splitter.

In summary, Howard is better than Splitter, but not all teams would be better with Howard instead of Splitter. Splitter is a system player tailor-made for the Spurs system. The things Howard does better than Splitter are things the Spurs as a team already do at an extremely high level with Splitter. There are probably only three teams in the league who would be better with Splitter than Howard, and those are all teams that a) already have scorers, b) play smart defense, and c) move the ball well. That's San Antonio and two teams led by coaches from Pop's tree in Atlanta and Golden State. Put Splitter on most other teams and his lack of athleticism and individual scoring ability becomes MUCH more of a liability, easily enough to outweigh his intelligence and skills away from the ball.