I have never read or seen any studies or tables that have been gleaned from sistering to show just how effective the process is.
I also feel that the amount of screws and the screw pattern has allot to do with the effectiveness of sistering and the support added. I also feel that sistering on both sides addes even more strength than on one side.
Sistering is only a patch to strengthen a long joist that has been compromised. Your comment about sistering a 2x10 to a 2x6 and having the floor above be fastened to this sistered higher board is NOT a good idea at all. In my thinking a sistered board is to support that joists but not to be the main joists or joists for a new floor being higher that the original joists.
This will not support it and will probably fail in time.