What effect does portafilter diameter have on espresso? It seems like a lot of the higher end espresso machines come with a 58mm portafilter.  Does this larger diameter have some improved effect on the resulting espresso?
MT San talks about portafilters and the resulting differences between these portafilters that are different in size. 

I will discuss the differences in espresso extraction as a result of portafilter and subsequent basket size.

Assuming that the only variable here is basket diameter:

Scientifically, pressure applied is normally measured as force per unit surface area. Taking the bed of coffee as a flat surface, and we assume traditional extraction pressure of 8 bar, the net extraction force upon the coffee puck will be greater for larger surfaces.

8 bar = 800kPa = 800 kN/m2

Now given two different basket sizes 58mm and 52mm which are 0.058m and 0.052m respectively, the net force on the surface of bed of coffee will be:

For 58mm:

Surface area: πr2 = π*0.0292 = 0.00264207942

Net force on area: pressure x area = 800*0.00264 = 2.11 kN

For 52mm:

Surface area: πr2 = π*0.0262 = 0.00212371663

Net force on area: pressure x area = 800*0.00212 = 1.7 kN

We can see that the larger portafilter is subject to a higher force of extraction with the same pressure. What this amounts to in practice is very much dependent on your setup. However it should tend towards better consistency in extraction. This is because, for a given dose, more of the coffee is in contact with the water at the bed surface and so has a more direct pressure gradient through the puck as it will be comparatively thin in the larger basket helping to reduce probability of channeling.

However, observable effects in practice will be slight to none as many other variables will take precedence in affecting shot flavour and the difference in extraction force will be accounted for silently in your dial-in process.