Regen depends on your definition of save.
Save for driving or save for the sevcon/ battery?
Battery wise there should be no problem.
The BMS allows up to 10,5kW of regen (8kW in older batteries) and the sevcon steps in when these limits are exceeded.(if you do not tinker wit the limiter manually)
Save for driving depends on the road condition and your driving abilities.
I think a suitable maximum amount of torque you can put on the regen is about 21Nm for neutral and 28Nm for breaking.
That puts the Twizy at the same amount a electro Smart will slow down without turning on the break lights.
The cars that follow you will see this as engine break.
You can shape the regen curve by using the TSMAP feature.