Ha ha this starter debate is never ending.
. FWIW a respected local starter rebuild shop told me that 616/617 turbo or NA starters are interchangeable. I have had starters from a 616, 617 turbo and 617 non turbo apart to steal parts from to put together a good starter and all the parts inside appears to be the same, i.e. they are not grossly different and are interchangeable. The starter that I cobbled up from used parts (I did replace the bushings and brushes and lubed everythig) is running perfect and cranks at 164 RPM per my phototach and starts my engine so fast it's incredible!
If you have a phototach and an ammeter, you can check out a starter on the bench (rpm and amps drawn) from Bosch specs. If it meet those specs, and there are no abnormal noise or smoke, and the bushings and brushes are good, slap that sucker in!
I think that theory of a 4 cyl and 5 cyl loads the starter 1 cyl at a time is flawed. At the very least, a 5 cyl has 1 more cyl and the cranshaft is longer and heavier so the starter has to work harder to move the extra weight so I don't know what the architect is thinking. My guess is Bosch made the decision to use the same starter for the 616 and 617 NA or Turbo engine because it is cheaper than to make different starters for each model.