Top speed is only one factor of a unit mobility even more so in the case of Case mates.
With out rotation and acceleration top speed does not say much.
Well since its job is to dive in frontally, shoot and drive backwards in the same way, speed matters quite a bit. Acceleration is in line with other tanks as well.
Stug-E/G both have superior rotation, mostly long range TD have worse rotation and they do not have to navigate the field to get in firing range. Another reason for it is possibly to make these TD more susceptible to flanks.
Generally speaking Brumbar does feel rather clunky to maneuver. It mobility I would say is inline with dozer 105 that even comes with turret.
The Dozer has some other stats that are quite different, mostly armor and HP (plus some vet later on). The Brummbar also has higher speed and acceleration for less rotation rate.
Regardless of the Dozer, all I was saying is that the Brummbar really has decent mobility for a heavy vehicle. Especially since it gets better with vet.