Hi Niels,
The AD5422 should be fine to operate at an update rate of 100kHz in the manner in which you describe, especially given the fact that you require small DAC value changes. Do you require 0.1%FSR accurate settling? Also, if you are only interested in an Iout part (no requirement for Vout), then perhaps the AD5420 would be better suited to your application than the AD5422? And cheaper too!
Rgds,
Tracey...