BMW doesn't make the transmission, ZF does. ZF sells several different 8HP models and all do with how much power they can handle. Do you know who else uses the ZF 8HP transmission? Chrysler. I seriously doubt those people will be going to a BMW dealer to get their transmission fixed or even an indy. There is nothing magical that a BMW dealer or indy will do.

Chrysler is building the transmission under license from ZF but they were buying them direct from ZF until the Chrysler plant in the US was producing them.

While you have the 6 speed model, it just shows you that BMW is not the only one using ZF transmissions. I have heard that Ford produced some ZF six speeds for their use. You have the ZF 6HP26 transmission. The Ford 6R60 and 6R80 is based on the 6HP26 design; how closely I don't know. But the Hyundai Genesis V8 4.6 from 2009 to 2011 uses the 6HP26, so there is a source of parts. 2008 - 2008 Lincoln Navigator also used it. 2005 - 2013 Land Rover Discovery used it. Various Jags also used it.