Shaft Length/Transom Height, Confused!

Presently I have Yamaha Outboard (Long Shaft) Model F15ELH mounted on a 20" Transom. In order to get the outboard installed at the correct height a Jack Plate was added. As you can see from the picture it looks good. Now the fun begins. I am planning on upgrading my outboard this winter. I am confused as to what shaft length I need. If I go with a long shaft 20" I will need the Jack Plate, even though the transom height is 20". Can I get a short shaft 15" outboard and take off the Jack Plate all together? What can I do to achieve max performance?

If you have a transom that is 15" high you need a short shaft motor.--------------If it is 20" high you need a longshaft motor.------------To me it looks like the cavitation plate in the picture is too high as it is.-----------------I do not think a short shaft will work ! but you could always talk to the mechanic at a shop and see what he / they recommend.