According to Bentanachs (2015), P. nocki differs from P. ruficollis, besides the color of the pronotum, by a sparser pronotal punctation and by more feebly punctured elytra. I have also doubted about the value of this new species : http://www.cerambycoidea.com/forum/topic.asp?TOPIC_ID=22464&SearchTerms=dubianella for me, all these differences are subjective rather than scientifically defined. I still leave the benefit of the doubt to the inventor of the new species because my P. nocki has a pretty deep blue color while my 4 P. ruficollis are greenish. I need more material to be able to decide !
Isn't that (the tarsal length and width) just a typical difference between male and females of many species?
Also, without the benefit of a series of material to study, I believe that it could be an individual variation. I have seen specimens of P. ruficollis much smaller than 27mm! There are some some specimens of ruficollis with very dark antennal segments (III to V), and some specimens in which those segments are very lightly colored.
The punctation which I see in a series of individuals of P. ruficollis varies. I have a specimen of "ruficollis" from Sabah, and the base and apex of the pronotum have a very slight amount of green color, like a hybrid of the two forms.