Probably it has something to do with optimizing the weight balance L vs. R - if it was hampering performance I am pretty sure they wouldn't leave it on there for the production car (unless of course there was no other way to construct the car)
And that last point might be the case: Notice that the pulley is only fixed on one side (the motor mount). The further the spur gear is away from the fixed side of the axle, the bigger the bending moment on the post that the axle is fixed to. This means more stresses, and even if the construction is strong enough, also more displacement. I would say that more displacement/flex (even with a rigid aluminium structure) easily puts the mesh those fined teethed spur gear and motor pinion out of line, and results in awkward belt tensions that you can't properly adjust the car for on the pit table.
If the situation above is indeed why Schumacher moved the spur gear close to the motor mount, then you can assume that optimizing the chassis' flex properties with a mount on just one side did more to the performance than moving the motor slightly further outside could hurt it, right?
