Just some thoughts on your plans:
Big ducer will loose bottom when on plane way before little one...
Big ducer costs twice as much as little one to replace
Big ducer to work above speed of 6 /15 mph has to be a little deeper in water as it pushes more water equals more air bubbles @ ducer so.... I would keep both and buy that split ducer wire they sell??? it hooks up the 200/83 from the little ducer (& temp) which U will use at slow & hi speed, and it also hooks to big ducer for Side Imaging and this way U can raise up big ducer somewhat to protect it from stumps & the like. Or see about using both as u may put them on each side of boat may not get too much interference.... I need sleep