Fundamentally, under the hood, there's only one reimplementation of the Windows APIs within CrossOver. The version of the bottle has two types of effects: 1) when a Windows program queries the version, they see the version of the bottle (or that configured using winecfg); 2) in the relatively few places where a Windows API functions differently on different versions of Windows, and where we've discovered applications which actually care about that difference, our reimplementation will mirror that difference -- behaving slightly differently depending on the version.
The performance of CrossOver is not affected by the Windows version of the bottle.
Many people assume that later versions of Windows are better, therefore a later bottle version would also be better. This is generally not true and can sometimes be exactly backward. If a Windows program detects a more modern version of Windows, it will assume greater functionality / more features are present, and it will rely on that. If the same program is run in a lower-versioned bottle, it will demand less of CrossOver and may be more compatible.