AndyC, you don't make sense. The header files just give you the function prototypes, and these prototypes do not change.
evildictaitor, Windows XP SP3 is supported until 2014. So this is in fact the first time VS drops an OS before it's support ends.
GetProcAddress will not affect performance if used right. That's why the CRT initializes itself, it makes the decisions once instead of deciding on every function call. For example in the new CRT during the initialization of the threading library GetProcAddress is used to get the right functions (different for Vista, W7, W8). It then stores these functions in function pointers, the decision is made only once.
You might think why the C++ guys complain about the new CRT not running on XP while .NET guys are quite silent about .NET 4.5 (which supposedly doesn't run on XP as well). That's because the .NET guys already have pretty much everything with .NET 4.0. The C++ standard was recently updated and it's a huge change, so much goodness. Not being able to use the new features is a disaster for us.
If you're a Microsoft employee (which I think you are), suggest someone from the CRT team to post on Uservoice to clear things up about this situation.