Any idea how much better full length sistering is than partial? I know there are too many variables to be exact, but running the full length in this case would be so difficult that I'd only try if a partial new joist would give just a minor fraction of the support of a full.
Also, the new joists will be added from the floor above. Can I sister 2x10's to the current 2x6's and have the old and new flush at the bottom (where the kitchen ceiling below is attached). This will make my floor upstairs higher, but I'd like the floor higher anyway.