> Compared to my proposition in issue #26382, this patch yields slightly better results for CPython 3.6, gaining an average of +0.36% on GUPB,
and similar results for CPython 2.7.

IMHO this change is too young to be backported to Python 2.7. I wrote it for Python 3.6 only. For Python 2.7, I suggest to write patches with narrow scope, as you did for the patch only modifying the list type.

"""
Table 1: CPython 3 GUPB results
-------------------------------
unpickle_list           22.74%
mako_v2                 9.13%
nqueens                 6.32%
meteor_contest          5.61%
fannkuch                5.34%
simple_logging          5.28%
formatted_logging       5.06%
"""

I surprised to see slow-down, but I prefer to think that changes smaller than 5% are pure noise.

The good news is the long list of benchmarks with speedup larger than 5.0% :-) 22% on unpick list is nice to have too!