I made a research about WPE and I have seen that below some distance there is not any difference between PMOS and NMOS devices, i.e. in both cases the voltage threshold increases.
But I am not sure if this conclusion is correct or if there are another properties that can be different due to its mobility carriers variation, stress/strain effects, ...
Proximity effect might be strain, litho field loading,
autodoping, etch interactions with previous cuts.
You'd have to dig deeper into a specific fabrication
flow, to say which (subset) are involved and probably
do this using physical material designed to exercise
these "features" such that they can be separated,
run through test flows which omit a doping but keep
the etches and thermal steps, etc. of one split and
contrariwise, on another.