Gouldjg, I've not a hydrophysicist, so I'll leave this one open. I've explained how I think it works, and you seem to understand well what I'm saying, but now I'm wondering if I'm wrong.
You know what I reckon, though? It's going to boil down to parameters, such as the mass of the submarine (and hence its inertia), the surface area and profile of the rudders, and stuff like that. Loads of variables. It's not going to be a straightforward answer, even if we can get hold of the knowledge we need to solve this one.
I'll add that both SH2 and SH3 model faster turn rates for faster speeds, and both sims have been careful to model the overall handling of U-boats as best as they can, and seem to have done very well indeed in that regard.
If you like, and if I have the time and energy, I could pose this question to a bunch of people who know this kind of stuff.