Paint on terminal was a problem not limited to WRX or
you have discovered the reason behind the existence of
the dialectric grease?

Poor grounding is a recurrent problem on some generations of subarus.

Some folks even buy expensive 'grounding kits', although all you need
to do is make sure what's there is in good shape, and has good metal-
to-metal contact.

Dave

Why did you decide to replace the cable?

Another possibility would be, disconnecting the battery might force the
ECU to return to the factory map. The rough running problem could return.

Carl