We now have a concrete use case. ;-) 
 
My idea was that we can introduce a new, CPython internal API that
aligns on 8-byte boundaries (or takes alignment as a parameter).  The
API would be a stop-gap measure. We can use the API to reduce
the overhead for specific types. 
 
E.g. for non-subclasses of float, we know the PyObject structure does
not need 16-byte alignment. We don't need a version of "alignof" to know
this. Inside floatobject.c, we could call the new objmalloc API that
gives new memory with 8-byte alignment. That would save the 33%
overhead. 
 
E.g. in PyFloat_FromDouble, rather than: 
 
 PyObject_MALLOC(sizeof(PyFloatObject))                                      
 
we could call something like: 
 
 _PyObject_MALLOC_ALIGNED(sizeof(PyFloatObject), 8)                          
 
This internal API would not be a permanent solution. Having to manually
fix each place that PyObjects are allocated and hard-coding the required
alignment is not the best solution. We can only fix specific types and
extension modules would always get the 16-byte alignment. Still, by
tweaking some of the most common types, we avoid much of the overhead
for the alignment change, at least for the average Python program. 
 
In the long term, we would need a better solution. E.g. an API that can
take alignment requirements as a parameter. Or, a solution I like
better, have types use PyObject_New().  Then, add an alignment
specifier to type object (e.g. tp_align to go along with tp_basicsize).
Then there does not have to be a new public API that takes alignment.