With a well designed coupler (which I assume they are), you don't lose much flow capacity. Although the coupler may have a small passage, it isn't very long. The flow loss is related to the velocity squared and the length. If the flow area is reduced to 1/2 the hose size for one inch of flow length in the coupler, the loss could be as small as adding 4 inches of length to the hose. It may not be intuitive, but you will gain a lot more by increasing the diameter of the hoses than eliminating the coupling, unless you are already pump flow limited.