My experience is a heavy 321 will have a higher DES speed (and climb...) than a 319. So most of the time, for a fixed ci DES, the target speed for all types will be a similar increment from green dot.
Today's 321 gave 291, yesterdays 319 gave 267 as DES speeds, both relatively heavy for their configs.
I agree, modulated idle in DES mode is higher than open DES, just when the speed runs away it should wind itself back to normal flight idle.