Piling in around a ring

I was recently perusing the work of one of the PhD students at LJMU and noticed some interesting outliers corresponding to pentahalo substituted benzenes in plots of property against clogP.  I was pretty sure that these compounds are likely to have issues relating to both axes.  With regards to the clogP values, most models will assume an additivity for these such that by and large they assume that going from phenyl to pentafluorophenyl is the same as adding one fluorine five times.  I'm sure some models will attempt to reduce this error with interaction terms but wanted to check out what the data actually look like.  This is for a series (taken from Hansch's book) in which F is added.  That source only has one value for each substitution type (none for F4) and these are plotted.

I have used AlogP here as x but you can see that this model has essentially even steps as you go from F0 to F6.  The measured values on y do not.  If you were to assume that F0 -> F6 is equal to six times F0 -> F1 you would assume that F6 has a logP of 3.0 rather than 2.6.
What about electronic differences?  The sigma meta and sigma para values for C6FnH5-n groups are:

F0: sigma meta: 0.06 sigma para: -0.01

F1 (para): sigma meta: 0.12 sigma para: 0.06

F1 (meta): sigma meta: 0.15 sigma para: 0.10

F5: sigma meta: 0.26 sigma para: 0.27

Again if the effects were additive, these latter two would be expected to be +0.51 and +0.54.  The fact that they aren't is curious.

So much for some of the problems with the x axis, I'm also curious by the y problem which is that you're plotting against a property with an innate assumption that it is similar binding modes in similar proteins that cause the effect - its a complex end-point in this case.  My hunch is that pentafluoro phenyl gets involved in qualitatively different kinds of interactions with other aromatic groups and cations.  Thus, such compounds belong to a different class to their less halogenated cousins and might be expected not to fall on the same line even given the problems in computing the descriptor values correctly.