Somehow you are feeling the back torque that normally occurs with a v-4. I'm not sure why it affects you so much, but the standard answers are there in throttle management, idle RPM setting, clutch release, and if available, the use of slipper clutches. Reprogramming to reconfigure off-throttle RPM roll back could theoretically retard engine braking. One would have that deceleration rate to live with in all circumstances, so I can imagine it would not suit all downshifting circumstances. The main purpose of downshifting is to be in the best RPM range in the power band to use for acceleration out of the apex in any case. As some wag has said, "The engine is not a brake", although it can become an expensive one.
I have tried killing the motor at speed and rolling throttle open or closed. There is a small perceptible difference in retardation, but not strongly so, not near enough to warrant all the fuss over it as a passive device. Opening a live throttle, on the other hand, has an enormous effect on deceleration. I'd shop there.