The effectiveness of a Hybrid system is directly proportional to the amount of regenerative braking it can provide. That is the ONLY source of free energy in a Hybrid. Charging the battery by running the engine or running it harder costs more fuel have than the stored energy will help you save when converted back to mechanical energy. That is because there is generation and motor losses along the way, not to mention a small amount of battery self-discharge. Everything else is secondary to this...
The Prius is a more effective hybrid than the 2nd Gen Insight because it can slow the car with a 67 hp motor. And (more importantly) it has 295 lb-ft of torque working to slow the car in lieu of the brakes and capture that kinetic energy into battery charge, whereas the Insight only has 13hp flywheel integrated motor-generator with 58 lb-ft of twist. Slowing a 3500 lbs Buick with a 15hp belt attached device which is torque limited by its means of attachment isn't much slowing and hence doesn't capture a lot of "free" energy for reuse. It's that simple. If GM is serious about eAssist it cannot be a bolt on Belt-Alternator-Starter. Is should be a motor integrated into the transmission or flywheel.
For a 3500 lb car... a hypothetical "Electramatic 6E75" will be ideal. Basically, it is a 6T75 6-speed automatic transmission with a 300 lb-ft input torque rating fitted with a 70hp/300 lb-ft synchronous motor in lieu of the torque converter. The car always pull away from a stop using just the electric motor with 300 lb-ft @ 0 rpm. At above 1000 rpm the internal combustion engine may start if needed. If the ICE is used the electric motor torque is electronically limited such that the combined torque going into the transmission never exceeds 300 lb-ft. During deceleration or below 1000 rpm the engine always uses the 2-stage Variable Valve Life system to shut-off all the valves and simply freewheel. This hybrid setup cannot idle the internal combustion engine; at rest the engine is always stopped and initial motion is always electric.
Hypothetical Electramatic 2.5L performance = 268 hp @ 4,700~6,300 rpm / 300 lb-ft @ 0~4700 rpm