As posted, Dash 8-400 avionics have nothing to do with -100/-200/-300 avionics.
The low maximum intercept speed used for the new system by Sextant (now Thales) might have to do with Bombardier giving rather conservative design specs, after all they were trying very hard to make the -400 as close to the older A/C as possible to allow for minimum training converting to the new plane.
(Which IMHO led to a lot of potential for avionics improvement not being used.)