The logic of going back to a BAS light hybrid approach in the LaCrosse escapes me. GM currently has a great full hybrid in the Malibu Hybrid, and this is a car using the same platform. The Malibu Hybrid uses a 1.8 liter ICE coupled to a slightly modified Voltec transmission. It has a 1.5 KWH battery and, I assume, the same inverter as the Volt to operate the Voltec transmission. This powertrain is capable of driving all electric up to 50 MPH and assists the ICE to feel much more powerful when it is operating. It is rated 48 city, 45 highway, 47 combined.
The new LaCrosse will have a 2.5 liter ICE coupled to a nine speed transmission. GM says it will get 25 city, 35 highway, 29 combined. The Belt/Alternator/Starter uses a puny 15 HP motor, so no chance of driving electric only and no chance of the smooth, torquey, drive characteristics of a Volt or even the Malibu Hybrid.
The key difference is the transmissions being used. I can't see that a nine speed transmission would be significantly cheaper than a Voltec transmission, not can I believe it can provide a more premium power delivery. Both vehicles have similar sized batterys and both use 3-phase AC synchronous motors (the Voltec uses two much more powerful motors) which requires an inverter to control motor operations.
What puzzles me is why offer a new, but less capable BAS powertrain on a higher end model when the Malibu Hybrid with better driving characteristics and efficiency is already available on the same platform? The Malibu Hybrid could also be expanded to a plug-in hybrid in the future by using the battery pack and charging units from the Volt. The BAS growth is pretty much limited to the belt's ability to handle power from the electric motor. I don't see it growing much more than the 15 HP it is now without resorting to a gear or chain drive which would add more complexity and cost.