Having driven a Chevy Volt for 4 years the topic of the efficiency of standard regenerative braking versus maximum regenerative braking (labeled Low driving mode in the Volt, also Chevy Bolt) was debated ad nauseam. When the GM engineers weighed in on this subject their response was that coasting (shifting into neutral) was the most efficient method of slowing the vehicle. Of course coasting is not compatible with driving on public roads in most situations. The next most efficient method of slowing the vehicle was standard regenerative braking recognizing that this is closest to coasting and converting forward momentum into electrical energy, storing the energy in the battery involves losses at each step of energy conversion.
Of the two levels of regenerative braking available in the Volt, the Low mode (actually provides maximum regenerative braking) was less efficient than standard regenerative braking as Low mode was furthest from coasting, incurred the most energy conversion losses. Finally, friction braking was absolutely the least efficient as all of the vehicle's forward momentum was lost when converted into heat energy.
Background: The Chevy Volt had true blended braking, something Tesla does not currently offer in any vehicle. In the Volt you can drive using one pedal, in Low mode, and this is similar to Standard regenerative braking with Creep mode enabled in the Tesla Model Y. The Chevy Bolt offers Hold mode, one pedal driving (but does not have true blended braking.)
Anytime a Volt owner attempted to quantity any difference in driving efficiency using the default Drive mode (low regen) versus Low mode (Maximum regen) they ended up splitting hairs in most driving situations. I don't imagine that this is any different in a Tesla. For example, if you drive the Tesla vehicle for an hour on the highway at 70 MPH regenerative braking, standard or low, will have minimum effect on driving efficiency.