In my opinion the under steer is a planed thing.
Cars with rear wheel drive are to be driven with a lot of discipline
So Renault added some toe in on the back(on my last 3 Twizys about half a degree) and put some smaller tires on the front.
Even with the track adjusted to zero you can't get rid of that understeering completly.
So I did put 145/70 on the front wheels.
No understeer up to 25 miles.
Only a slight tendency to over steer when you drive it to hard.
But with the original contis you can hear when the tires begin to loose traction.
Since this year I also have 155/70 fitted to my back wheels.
They are a little smaller (3%) and shift the balance a little to the back.
with this configuration the Twizy runs like on rail road tracks.
I tried very hard to get it to drift (Got a OVMS with 18,5 kW setting )