I'm pretty sure that GM have been doing their own battery pack testing, R&D and integration for quite some time now, even partnering with a local university to

I'm pretty sure that GM have been doing their own battery pack testing, R&D and integration for quite some time now, even partnering with a local university to train people on this. The main component they outsource which Tesla develop in-house is the motor and its controller, and that's obviously pretty vehicle-specific.