I have the sealed floor on my 156D. I am not sure if you are going to gain any significant maneuverability that is worth the cost. Sealed floor is great for unsticking a rock or light portaging. I also like not having to worry about cleaning out sands and small rocks, but for maneuverability, you may have a better result by how you balance your gear load. My boat is pretty quick to turn, much faster than any other boats I've rowed including Sotar, but, 16' raft are not really for speed. When I am on the water, I am always lagging behind 14' or cats with little gear. I actually enjoy rowing a larger gear boat. It takes a lot more planning to move the boat especially in narrow rocky places.
See if AIRE has any used sealed floor. I bought mine with the sealed floor and have no regrets.