The word "toolset" is a bit of a misnomer to describe the differences between v110 and v110_xp. You are still using the same build tools. And you still have the same version of the CRT. Something you can see by comparing what you see in the Debug + Windows + Module list of loaded DLL between the two builds. Note the name and location of msvcr110.dll.
The CRT was in fact updated by Update 1, it now supports both XP and newer Windows versions. This works by it binding dynamically to the later winapi functions at runtime, using GetProcAddress(), limping along if it can't find them when you run on XP.
What is different is that you got another version of the Windows SDK. The last one that was still compatible with XP, version 7.1. You'll find it back in C:\Program Files (x86)\Microsoft SDKs\Windows\v7.1A. When you build with the v110 toolset then you'll use the SDK include and library files stored in C:\Program Files (x86)\Windows Kits\8.0
The specific changes when you use v110_xp are visible when you search the c:\program files (x86)\msbuild directory for that string:
- the directories for include and lib files, etcetera, are changed to the Windows 7.1 SDK paths
- the _USING_V110_SDK71_ preprocessor symbol is added, not otherwise used anywhere important
- the linker's /SUBSYSTEM option is altered to require only Windows version 5.02, the XP version number.
Long story short, mixing modules that were built with a mix of v110 and v110_xp toolsets is not a problem.