At the very least, this is a difficult passage. One could wish Paul had given some sort of clearer explanation as to what the Holy Spirit was telling him.
Since Paul used the phrase "when he has done this", it speaks of a future time, otherwise, he would have said, "when he HAD done this....". This passage is not only speaking post resurrection, but post redemption of the saints, since it's speaking of abolishing the "last enemy" in verse 26 (which is death). We have not yet seen death abolished, so it is not speaking of the here and now.
The Trinity is a mystery and I often feel our human minds have great difficulty with it. As one pastor once told me, "you're crazy not to believe in the Trinity, but it will make you crazy trying to understand it". Amen. The Son, I believe, even though equal with God, subjects himself to the Father, just as He did on earth. At this point, there would no longer be a need for a Godhead in 3 persons, which is what I believe is meant by "so that God may be all in all".
It is a mystery not easily explained in human terms. I'm too tired to do much more research tonight. I will try to look into this and edit my comments if necessary. I'm also praying for clarity from the Holy Spirit.
Added 12/2: The author of I Corinthians is Paul, so I see no problem using any Pauline epistle to illuminate a passage in another Pauline epistle. In Philippians 2:6-8, Paul states,
"Who, being in very nature God,
did not consider equality with God something to be used to his own advantage;
7 rather, he made himself nothing
by taking the very nature of a servant,
being made in human likeness.
8 And being found in appearance as a man,
he humbled himself
by becoming obedient to death—
even death on a cross!
If Paul would describe Christ as "being in very nature God", I think we can lay to rest the argument against the deity of Christ. That does not, in and of itself, answer the question presented in I Corinthians 15:28. Based on verse 24, "Then the end will come", this is obviously when there is a new heaven and a new earth.
If you feel you need to conclude, from this single verse of scripture, that Christ is not God in the flesh, so be it. I myself will continue (based on a plethora of other scripture verses) to believe that He is and will continue to believe He is King of Kings, Lord of Lords, The Great Physician, and the The Lamb of God. To Him be the glory! Amen.