It appears there are some misconceptions surrounding the use of regen and how it works with the Kia Soul EV. It would be worth your time to select the EV display and change to the screen where you can see the power that the drive system is using.
While driving and when safe to do so, watch the power value. Note that as you go from steady state driving and slowly let up on the throttle you will see the value decrease, eventually hit zero, and then become negative. Do this in both the D and B mode. Pick a speed and notice how much max regen there is when in D mode and you fully let off the throttle and then compare the same scenario while in B mode.
Also, notice what happens to the regen value when fully off the throttle and you slightly press on the brake pedal. Only press it enough that the brake lights would come on but the friction brakes don't. There is a very slight increase in the regen power. Note that even if you press the brakes harder that the max regen doesn't increase. This can take a bit of testing because the value is constantly changing. The amount of regen doesn't appear to be tied with how hard the brake pedal is pushed aside from the small initial increase when the brake lights would have come on. This is true in both modes. Because of this and the fact that regen isn't a fully off or on event it is more efficient to drive properly in the B mode than in D mode.
As for using neutral to coast down a hill at a higher speed than using regen to maintain speed being more efficient, that depends. The faster you travel the more of your potential energy is lost to air friction. That difference in air friction between say 60mph and 70mph could have been turned into recovered energy if the speed were kept at 60mph which then would have been used to propel the vehicle the extra distance that would have been coasted by the higher speed. This is especially true if the terminal speed was 70mph. Most of the way down the hill air friction was keeping you at 70mph but you only benefit from the extra 10mph of roll-out at the bottom of the hill. It would have been better to regen down the hill at 60mph then use the regenerated energy on the roll-out that the 70mph speed would have given. This is exactly the method I used driving from the charge station in Castle Rock, WA to Coldwater Lake, up Mt. St. Helens. The trip normally can't be done on a full charge so driving slower on the way up helped but still didn't have enough range to make it back to the charging station. By driving only 45mph and using regen on the way down there was ample range. In any case, without hard data for a given situation it is difficult to make a claim either way.
David D. Nelson
Blue 2016 Soul EV+ & Cloud 2018 Soul EV+