I seriously doubt that the coil is the issue with your problem. Idle is one of the lower demands a coil faces.

I also don't know of any 300Es with (ignition) ballast resistors. For the most part changing the capabilities of one's coil does absolutely nothing to driveability. The coil only produces the voltage requred to jump the highest form of resistance. The energy stored in a coil has to do with the current used and the dwell time. If you alter the current you stand a good chance of frying the driver circuit in the controller. I'm pretty sure than MB uses a current limiting technique in those modules and as such if limited to less than the designed operating current the output might even be worse with a generic coil.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician