What size 20's are you planning? I have ACS springs, 20" 312's and space saver tire - the spare is about 26.5" overall the same as my 275/30/20 rear tires. Yes you will sacrifice some trunk space. Get a good tight fitting cover and rachet tie down down strap and you're good.
F10 550i Space Gray | Dinan Stage 5 | Eisenmann Race | ACS springs | 20" Ferric Grey 312 - Michelin PSS