John 20:28 is a controversial passage that has been explained in a sundry and diverse ways. The brilliant exegete Theodore of Mopsuestia (one who affirmed Christ's Deity) felt that John 20:28 was not addressed to the Son, but to the Father of our Lord and Savior. Of course his approach was condemned in 553 C.E. and does not have wide acceptance today, though it is certainly a probable view at the very least.
Frankly I have no problem with calling Jesus KURIOS or QEOS. Yet this doesn't mean that Jesus is Almighty God. In Col. 1:15, 16, human and angelic rulers are called KURIOTHTES. Acts 2:36 also tells us that Christ was made KURIOS--he was not always such. Yes, he was QEOS in his pre-human existence, but I think that John makes a vital distinction between the Almighty hO QEOS and the one who was QEOS (without the Greek article).
Scholar J. Gwyn Griffiths writes that QEOS in Jn 1:1c, "Taken by itself" could be rendered "And the Word was (the) God" or "and the Word was (a) God" (See "The Role of Theology and Bias in Bible Translation" by Rolf Furuli). Many students of the early fathers also know Origen thought that God with the article denotes something different than God without the article in Jn 1:1c.