I wouldn't do it b/c I wouldn't want to give up my convertible, and since you live in sunny SoCal, it would seem like a convertible would be a must.
You can do a search to find all the usual performance comparison points between M6 and six series, but since you mention CPO and extended warranty in your OP, it would seem to me that you're better off sticking with your 650 due to increased repairs, maintenance and operating costs (gas and insurance) of the M6. Others on this forum have owned both and can chime in.
'07 650i vert, beige/jet black, sports & cold weather pkgs, comfort access, logic 7, sat radio, Miro M6 wheels (with Michelin super sports), M6 spare tire, tint, windscreen, triple resonator delete, m-tec v3 AE's bulbs, corner bulb delete and m-tec fogs
'11 Acura MDX Advance with Entertainment pkg for wife - no mod's
'04 Acura TSX (daughter) and '12 VW GTI - Autobahn (son)