I'm not an expert, but I suspect some Newtonian physics gets you close enough to an answer...
The force needed (wheel torque) to accelerate the car is equal to the mass of the vehicle (including passengers) times the rate of acceleration. f=ma
Adding 400 lbs of weight to your LEAF will increase the total mass by about 10%. Since most of the pack energy is used for acceleration of the vehicle, then expect a 10% increase in energy use (so a 10% loss of range) while accelerating. When doing regen, you gain some of that back, but Nissan limited regen on the LEAF so not as much as one would hope for.
Vancouver, CA owner of a 2013 Ocean Blue SV + QC, purchased 01/2017 in WA
Zencar 12/20/24/30A L1/L2 portable EVSE
1-1/4" Curt #11396 hitch
After market, DIY LED DRLs
LeafSpy Pro + Konnwei KW902 ELM327 BT OBDII dongle
Loving my first BEV