Christoph, your experience with Silverfast and mine are similar. Using aRGB solves the profile mismatch problem. Of course, as you already mentioned, using aRGB means that you may not get all the colors your scanner can produce.
A while ago, I did a few general scanning tests. I was just trying to determine which scanning software produced better negative scans with my scanner. My tests were not intended to compare gamut. I randomly selected three photos. To my surprise, one of them showed gamut clipping with Silverfast and aRGB compared to my scanner’s native color space. Converting from my scanner’s native color space to PhoPhoto didn’t show any clipping.
BTW, the photo wasn’t one where I would have expected a gamut larger than aRGB. It wasn’t a colorful sunset or saturated flowers. However, I didn’t print the photo, so I don’t know if the gamut differences would have been noticeable in a print, but I suspect not. In any event, I know you’re fully aware of gamut issues. Just thought you’d be interested in my experience.