I pulled one of these starters apart this week.
Older models, I think pre 2008 or so, used the Valeo starter. These suffer from bendix issues, but very little else. The bendix can be replaced with an aftermarket unit, although the dimensions of the aftermarket bendix are a little different and some precision machining is required.
Later models have the Bosch starter, which is the one I pulled apart. I found, and this is not the first time, that the brushes appear to be very soft and start crumbling. This is what the brushes looked like.




The contact area has also spread out to the point where the brushes are making contact with more than one copper strip on the armature head, which IMO is what is causing the sluggish response. Once the brushes were replaced, starter was good again. I also replaced the small DU bush in the nose of the starter.
This is the finished product.