Energy captured through regenerative braking can be expressed as (-) kW as it is power that is being sent back into the battery. If, just for example, Low regen has a maximum power level of -40kW and Maximum regen has a maximum of -80kW then if we apply Low regen for 10 seconds to slow from 60 MPH to 15 MPH and Maximum regen for 5 seconds to slow from 60 MPH to 15 MPH the amount of total energy that is captured is going to be the same.

Other factors include that Low regen will experience mechanical energy to electrical energy conversion overhead losses for twice as long. Maximum regen will experience higher current, increased losses due to resistance as heat losses but over a shorter period of time.

Examining the vehicle's overall efficiency includes how far the vehicle would travel during regenerative braking. For Low regen, assuming starting at 60 MPH and slowing to 15 MPH, I estimate the vehicle would travel a total of 550 ft in 10 seconds. For Maximum regen the vehicle would travel just 275 ft in 5 second. Assuming 250Wh/mi energy consumption when driving at 60 MPH the extra 275 ft traveled represents ~5% or about 12.5Wh. Over the additional 5 seconds this is insignificant at less than 2W but it does add a small amount to the vehicle's overall efficiency while using Low regenerative braking versus Maximum regenerative braking.

I don't have the source but studies have been performed that show that regenerative braking can recover, convert up to ~70% of the vehicle's momentum and store this energy in the battery.