There are so many variables and driving styles no one answer will work for everyone. However, one thing is inescapable. There is more energy lost in slowing down through regen then using electricity to accelerate than there is through careful coasting without regen or accelerating.
Which is why the obsession with heavy regen is actually counterproductive. I believe this must be the reason the nonsport modes automatically reset regeneration to the lowest levels.
Whether in EV mode or HV mode, you will notice on the power/charge gauge that Clarity tries to recoup the loss from deceleration into battery charge. And there seems to be no difference whether the deceleration is caused explicitly by braking or implicitly by paddle regen. A feature of the paddles is that they allow different degrees of deceleration. And also support "one pedal driving".
In my opinion, allowing stronger decelerations is a more consistent feature of SPORT than of ECON, and is not as supportive of energy efficiency, as The Gadgeteer has eluded to. Nonetheless, I think it is a great feature for those coming to the Clarity for the first time, who will be thinking: What kind of Volt is this!?
Actually you can have both EV and Sport mode on at the same time! Your car can be driven in ECON, NORMAL or SPORT. Each of those modes can be in EV or HV. In addition you can drive in NORMAL or SPORT with HV CHARGE.