I take it back, my methodology in reproducing the function calls used by the gzip module was flawed. 

It does look like a bug in the CRT, but I have not been able to isolate a simple way of reproducing it. I have however, found a workaround for it, that has an acceptable performance impact. 

https://github.com/kovidgoyal/cpython/commit/72ae720ab057b1ac0402d67a7195d575d34afbbd

Now all tests pass (except for tcl/tk and distutils, neither of which I care about -- well I will probably need to fix up distutils at some point, but not now :). Running testsuite as

./PCbuild/amd64/python_d.exe Lib/test/regrtest.py -u network,cpu,subprocess,urlfetch

@steve: Thank you for all the work you did porting python 3.x to VS 2015, that certainly made by life a lot easier.

I would of course, be ecstatic if you were to consider merging my work into the python 2.7 branch, but if not, I understand -- no one likes to maintain a legacy codebase.

In any case, for interested third parties, my work is available here:

https://github.com/kovidgoyal/cpython (2.7 branch)

and instructions on building python on windows using a nice cygwin environment are here: 

https://github.com/kovidgoyal/calibre/blob/master/setup/installer/windows/notes2.rst