You are 100% right about Ⲫ and Ⲑ
in sahidic dialect they are pronounced p-h and t-h
but this is not the case in Bohairic
I have read the book you mentioned, but each book gives different pronunciations for some letters
Sahidic dialect was dead long before the scientist start paying attention to Coptic Language
I am not sure, how or if the authors of these books really know the right pronunciation
if you have any scientific evidence of any kind regarding to the pronunciation of Ϫ Or Ϭ (or any other letters) I would be glad to see it
but regardless of all this issues, pronunciation of a letter in sahidic dialect isn't always the same as its pronunciation in Bohairic dialect
for example the letter Ⲃ is "f" or "b" in shaidic but "w" or "b" in Bohairic and so on
