unfortunately every change will involve a compromise. Lowering your warm idle to 650 will probably cause it to stall when cold as the cold idle will be under 400rpm.
I would remove the restriction and plug in the ECU and see what happens. If you get the uncontrolled max idle then remove the ECU and have an electronics tech resolder the PC board. Try again. If it don't work get back with us and there are instructions for diagnosing the electronic control.
Bosch Master, ASE Master, L1
33 years MB technician