You are correct in the channel power increasing 3db. This should work with the setup that worked with 6.4MHz wide modulation. No adjustments needed in combining just optimize LASER performance if needed. If your carrier was 1.6MHz QPSK you would be at 17-18. You sound like you have everything in order and should not have any issues. Watch your spacing.
This is CABL.com posting #368501. Tiny Link: cabl.co/mbH1J