I have seem several 9" wheels between +36mm~+50mm for your car, but not any 20" wide. They are all 17" & 18", but diameter won't matter in reguards to your question. Can you put up a link to the wheels said to be +24mm?
I would recommend you go with the +40mm to be safe. I believe the +24mm would be too far outward for either end of your car. I figure you are looking for a somewhat staggered look and push the rears out as you have additional room back there. I would recommend if you REALLY want this look, stick with the +40mm wheels and upgrade your lug-bolts to longer/stronger ones and run some 1/2" spacers to move them outward in back. This is a lot of work, but can install a spacer to move the wheels to exactly where you would like them and the upgrade of bolts will be stronger than the stock ones you run now.
LMK what you think
Jazzer
