The WD car used a two-motors-in-one-housing unit to get the most power it could (
link here), but even that only had 120hp peak, so the Leaf motor is quite impressive. The power curve for their motor looks just like a 40V S8's torque curve, but the torque for the motor looks like a steam engine
AC-35x2 Torque Curves
68Kg for that, so 80 for the Leaf including the reduction gear isn't bad. I reckon it would work with a 4:111 reduction based on the graph. Its just like being in 4th all the time which would be fine since you're not going for top speed. Using the 3.7 final drive from the 4.2 A8 would probably be better actually since you'd make better use of the torque spread up to 3000rpm - 64mph, vs 58mph with 4:111.
Koenigsegg can do 0-248mph with no gears using a torque converter and an MGU on the crankshaft!
https://www.youtube.com/watch?v=glf_k4qGBAA. They're effectively in 7th gear all the time!
Controlling two motors with aftermarket controllers appears to be simple - one controller for each motor, both being fed the same speed signal input. Thats what the WD car did, albeit both motors were built in to the same casing. It would be interesting to see if the control device talks can-bus, then you could still run the ESP
You'd need it with all that torque.