I’m curious as to why in the east the epiklesis is regarded as the time of transubstantiation (or since we’re discussing eastern thought,
metastoicheiosis 
) rather than the institution?
The Orthodox consider that it must be explicit, as in the Liturgy of St. John Chrysostom. For their Western rite, they add the explicit epiklesis from the Liturgy of St. John Chrysostom.
Transubstantiation does not require the explicit epiklesis, according to the Catholic Church, as stated at the
Council of Florence and reiterated at the
Council of Trent and in the statement of
Pope Pius X (1911):
“But the Catholic doctrine on the most holy Eucharist is not left intact when one resolutely teaches that it is possible to hold the opinion which maintains that, among the Greeks, the consecratory words do not produce their effect, unless that prayer which they call the epiclesis, has already been offered.
For it is certain that the rights of the Church in no way make her competent to alter the substance of the sacrament in any respect …”
The Eucharistic Epiclesis: A Detailed History from the Patristic to thet Modern Era by John McKenna, p 89.
It was also stated recently that The Anaphora of Addai and Mari (Assyrian Church of the East) is notable because, from time immemorial, it has been used without a recitation of the Institution Narrative, yet it is valid (this decision made 2001 CDF, approved by Pope John Paul II). Note that they
do use an epiklesis.
“The Catholic Church considers the words of the Institution as a constitutive part of the Anaphora or Eucharistic Prayer. The Council of Florence stated “
The form of this sacrament are the words of the Saviour with which he effected this sacrament. A priest speaking in the person of Christ effects this sacrament. For, in virtue of those words, the substance of bread is changed into the body of Christ and the substance of wine into his blood” (D.H. 1321). The same Council of Florence also characterised the words of the Institution as *“the form of words [forma verborum] which the holy Roman Church …] has always been wont to use [semper uti consuevit] in the consecration of the Lord’s body and blood” *(D.H. 1352), without prejudice to the possibility of some variation in their articulation by the Church.”
vatican.va/roman_curia/po…assira_en.html
(Cardinal) Metropolitan Bessarion of Nicea, 1439:
“And since we hear from all the holy doctors of the Church, especially from blessed John Chrysostom, who is very well known to us, that it it those words of the Lord which change and transubstantiate the bread and wine into the true body and blood of Christ and that those divine words of the Saviour contain all the power of transubstantiation, we ourselves, by necessity, follow this most holy doctor and his opinion.”
Then in 1442, in the Decree for the Jabcobites, the explicit “verba Salvatoris” is stated as “Hoc est enim corpus meum” and “Hic est enim calix sanguinis mei…”.
The quotes above are also from The Eucharistic Epiclesis: A Detailed History from the Patristic to thet Modern Era.