My understanding is MGA is the smaller motor, which is clutched to the engine in Range Extended mode to generate electricity "on the fly" as fuel for MGB, the primary traction motor.The max I ever observed on my Gen 1 2013 Volt was 59kW, while braking from 55mph. Theoretically, it should not be higher than 55kW (which is what MGA/generator is rated)
Braking regeneration is created by MGB using the car’s momentum when this motor is not providing propulsion torque to the wheels (e.g., when coasting in D or L, slowing down, or being pulled downhill by gravity).
When comparing recharging rate potential, note that Mountain Mode is capable of using MGA to recharge a fully depleted 2012 Volt battery to the 45% MM-maintained level (~4 kWh) in 15 minutes, using 0.36 gallons of gas (see the Self Charging Volt youtube video).
I would think max regen rates are not normally sustained for 15 minutes or more.