Undoubtedly R J Mitchell applied important lessons learned from his racing aircraft when he designed the Spitfire. However, the similarity between the two is only superficial.
The fuselage of the Spitfire is made up of sections bolted together with a separate tail unit, whereas the S.6 seaplane had a fuselage built as one piece including the large tail-fin. Amazingly this acted both as oil- tank and oil-cooler, from which lubricating oil was pumped back to the engine via further oil-coolers on both sides of the fuselage. Such an arrangement would put a warplane at risk from a single bullet.
The beautiful cantilever wing of the Spitfire is an integral part of the airframe, strong enough to withstand the speed and stress of combat, as well as thick enough to accommodate eight machine-guns besides the retracted undercarriage. The S.6 used a thin wing of uniform chord and section which had to be heavily braced with wires extending downwards to the huge floats and upwards to the fuselage. This wing doubled as a radiator.
Mitchell's outstanding success with two very different designs for two very different purposes puts him among the greatest aircraft designers of all time.