Greco-Bohairic (Modern Ecclesiatical Pronunciation):
In essence, linguistics is a descriptive field. It does not say which pronunciation is 'right' or 'wrong'. Pronunciation is essentially a code. However, there are various issues that arise on abandoning the native tradition, with its histrocity. I'll quote the opinions of Coptologists for that matter. Here are synopsis of commentaries by Coptologists who were interested in the field of Bohairic pronounciation.
About Bohairic (OB), as the only dialect used in Egypt & about the term Sahidic >"Bohairic is the only dialect known to the present-day Copts.."
"The term <Sahidic> (Sa'idi) nowadays is reserved exclusively for the despised <old> pronounciation of Bohairic, as heared particularly among the peasantry of Upper Egypt" (W.H.Worrell, Coptic texts Ann Arbor 1942) About the comparison between the Old Bohairic (OB) & the new one GrecoBohairic. >"Among the peasantry of Upper Egypt, their survives in ceratin places a family tradition about the pronounciatio of Coptic, which though extremely meagre, is genuine & superior to the pronounciation of the clergy emanating from Cairo. They have great pride in the posession of a family tradition & its superiority" (Worell, A short Account of the Copts, Michgan 1945)
About the discovery of W.Till that all the Coptic liturgical tradition is Bohairic & not Sahidic >"Till, in 1929 reached the correct conclusion that traditional Coptic pronounciation is at best Bohairic not Sahidic" (Worell, Coptic sounds Ann Arbor 1934)
About the Greco-Bohairic naming it reformed pronounciation & the superiority of Old Bohairic over it. >"The reform pronounciation has introduced errors & confusion"
"Reform should have been in the direction of the peasant tradition, but that was too much to expect. Instead, Modern Greek values were introduced systematically, & these values are unfortunately being taught by those who are backing the very creditable enterprise of reviving the Coptc language in Egypt. The old tradition is now to be found so far as I know, only in Upper Egypt, or among those who have come from there & who have not yielded to the pressure of the cities (Worell Coptic Texts Ann Arbor 1942)
About the introduction of Modern Greek pronounciation to the Bohairic >"The introduction of the Modern Greek into Coptic nowadays is often an affectation: as though one were to pronounce all the French words in English according to the present usage of Paris. If this is bad, how much worse it is to pronounce Coptic according to the artificial Erasmian Greek system which belongs to no race, age place or dialect..." (Worrell, Coptic Sounds)
"It is not necessary to suppose that the Copts took over Greek letters with exactly their current Greek values but only their approximate ones. Even sounds ordinarily identified in two lnaguges are rarely actually identical. That is the case, for example when Persian or Turkish is written in Arabic letters. It is not likely that Coptic and Greek vowels were identical" (Worell, Coptic Sounds)
About the Greco-Bohairic in educational books >"All modern books written on Coptic by native authors adopt more or less a mutilated form of Greek pronounciation & apply it entirely to their language. Unfortunately, none of our native authors here knows sufficient Greek to realise the outstanding mistakes he is trying to form into rules applicable to the Coptic language" (Georgy Sobhy,Journal of Egyptian Archaeology 1915)
About the superiority of authentic Bohairic (Old Bohairic) over the Greco-bohairic pronounciation. >"I believe that an ordinary uneducated priest in reciting any Coptic prayer in Church pronounces the language much more correclty & naturally too than if he followed these erroneous rules set down in the modern Coptic books" (Georgy Sobhy,Journal of Egyptian Archaeology 1915)
About the superiority of inherited Coptic pronounciation (old Bohairic) over the Greco-Bohairic >"Its unbroken use in the Church has undoubtedly preserved its pronounciation, for it has been handed down from one generation of priests to another until our days; and in my own belief a priest who has learned to pronounce this language from his predecessor without the use of the modern sophisticated rules exhibited by Coptic authors in their writings does inherenlty pronounce it more rightly than any other man" (Georgy Sobhy,Journal of Egyptian Archaeology 1915)
About the introduction of the mutilated Greek pronounciation >"The soundest observation that Sobhy has made is that the introduction of an ignoranlty mutilated Greek pronounciation is to be depreceated" About a text dating to the 10th Century were Arabic was written in Coptic letters, & comparing pronounciation of the 10th Century to the one used at the churches (Old Bohairic) that was used excessilvely at that time. >"The importance of these leaves is paramount. They teach (us) first of all how Arabic was pronounced in the period when the manuscript was written ... They also teach us that until the period at which the manuscript was written Coptic was still the language spoken by the monks & the Coptic church at least in Monasteries. Last of all, the most important result of studying these leaves is the knowledge we gain of the values of different letters in the coptic alphabet and their equivalents in Arabic. It corresponds most closely with the actual pronounciation of Coptic in the Church" (H.G.Evelyn White, New Texts from the Monastery of St.Macarius NY 1926).
About a manuscript dating to 1722 written in coptic & arabiczed Coptic (Coptic written in Arabic letters) >"Here lies the value of the book which shows us how Coptic was universally produced in Egypt in the Early XVIII century A.D. & when we compare it with the earlier manuscript & with the manuscript of St.Macarius which was Arabic in language but Coptic in lettering we can hardly detect any difference whatever in the phonetic values of the letters or in the way they were pronounced. Going back still further in time we can, from the documents we have, prove the stability of this pronounciation through the ages. Even the glosses written over the demotic words in Greek letters in the demotic magical papyrus of London & Leiden show that the pronounciation in our book is practically the same as demotic of the above manuscript, which brings us back to IInd Century A.D. (Georgy Sobhy, Tradional Pronounciation of Coptic in Church of Egypt, 1940).