cant believe i actually read this whole thread and only an odd post here or there replied to the original question
imo all these top players are pretty much all-rounders, their preferations might be just minor - yes, it's probably due to surface homogenization - times when the great clay champions, likes of gaudio, got allergic to grass at wimbledon or the world leader (hi, pete) was losing to delgado or blanco at RG are just gone.
this is why i think about the clay specialists we can talk only about these lower level players, who still perform on carpets of sarajevo or clays of monza
for instance, alessio di mauro - would you believe this italian ex top 70 player has just performed this week in his first hc tournament since 2007?
these specializations would typically start below the top 30 with guys like andujar, bellucci, zeballos, berlocq, but even they have their feast in miami or to smaller extend indian wells of today. fognini, montanes are also clay specialists