Manual targetting is as effective as units scatter.
If you nerf manual targeting, you're nerfing autofire equally as well.
There is no power spike with manual targetting, manual targetting simply assures the squad is centered in the scatter box while autofire can put it at the edges of the squad due to targetting models on the sides.
The power spike is the shells actually hit.
With the Brummbar, the shells are so slow that if you fire at a moving target, they'll barely hit at all. The targeting algorithm doesn't lead shots, so you have to manually target them to hit.
This isn't a problem for units with faster shells because the travel time is that much shorter.
Speeding up the Brummbar's shells to a speed more like the KV-2 mitigates that. It doesn't buff manually targeted shots (which are led), but closes the gap between its manually targeted performance and its autofire performance to something more comparable to a normal tank.