In other Words......I realise that when I unfuse my Level 80 white Kyurem, my Reshiram is back to level 70 while only Kyurem is at level 80....Should I train Reshiram to level 80 then fuse them back for best stats or it doesnt matter?
The levels, EVs, IVs, nature, and all that come from Kyurem. Just use a Level 1 Reshiram with a Level 100 Kyurem, and you'll still get a Level 100 Kyurem-W with all the EVs, IVs, and Nature of the original Kyurem.