"In order to maintain control and improve administration, various schemes to divide the work of the Emperor by sharing it between individuals were tried between 293 and 324, from 337 to 350, from 364 to 392, and again between 395 and 480. Although the administrative subdivisions varied, they generally involved a division of labour between East and West. Each division was a form of power-sharing, (or even job-sharing) for the ultimate imperium was not divisible and therefore the empire remained legally one state—although the co-emperors often saw each other as rivals or enemies rather than partners."
That is probably the best evidence that the cost of communication rose to be impracticable; the Emperors divided the empire because it was not possible to exercise imperium.
I think it would be interesting to (a) examine the rise of the proconsul as an adaptation of the Roman Republican system to the increased cost of communication and (b) the general question of how the Roman Republic balanced delegation of power against the difficulty of maintaining oversight (I'm going to assert that within the context of your question the value of communications is the exercise of oversight) - but those questions are far beyond the scope of stack exchange. (
