I agree. I would not buy the 56 with the thought you may grow more. You may not, and even if you did, the 56 would not be necessary now. You are comfortable now, that is what is important. Even if you grow to be 5'10" the 54 will fit. You may need a longer stem, but that's about it. If you grow to be 6' 1" then you will need a different size, and most likely larger than a 56. That's when you sell the Diverge and buy a larger bike.
Enjoy the new bike.