Are you wondering about hydrodynamic effects on the leeward side, or aerodynamic effects on the weather side?
In the first case, if you're heeled over that far you have other problems. In the latter, I can't imagine it would have much affect on windage.
I am wondering about the hydrodynamic effects. For instance, does pronounced tumblehome produce a lift to leeward? Or maybe it helps the leeward hull side "stick" in the water and reduces leeway?
The boat with pronounced tumblehome I have sailed on is a Catalina 38 which points and goes to windward very well, and buries the rail (and of course the tumblehome) when there is sufficient wind. It seems to like that.
The tumblehome was part of an S&S race design as I understand it, so I am assuming it was done very purposefully. But why?