to me it is the carving in the marble and its clearly modeled relief, as "plastica" has the sense of modeling and "resa", not yield but here would be "clarity of the relief" or the quality of the carving, yet to me "modeling relief" doesn't sound very good, so I have reduced it to my answer, as relief is defined as precisely such: from the dictionary - a method of molding, carving, or stamping in which the design stands out from the surface, to a greater ( high relief) or lesser ( bas-relief) extent.

it's more pretentious arty talk...the next piece of airy-fairyness would be "risalta con limpidezza"....

"Resa plastica" indicates the level at which the material has been worked to successfully convey a sense of tridimensionality (modelling) and detail (notice how the writer characterizes it as "excellent") - therefore it pertains to the sculptural quality of the architecture. However, I would translate it here as "plasticity" which is an English noun, and used, for instance in artistic terminology of the 1930s - 1950s (especially in the writings of Clement Greenberg). I would also keep the "resa" by translating the verb "risalta" with "rendering/is rendered" or something similar that manages to retain and to justifies the human skill it took to successfully bring to life (render) an otherwise banal ornamentation.