Where a ballast would show up problems, if it simply was wrong resistance, is heating or failure of the coil or ballast, or missing at high speed if too high resistance.
FWIW.. I have found the exact opposite.... crappy low RPM operation, especially at idle and when warming up. IMHO, the OP needs to see if he has the right parts in there.
I have a /6 where the wrong ballast was put in there by the P.O., and it had idle problems all the time... its resistance was something like 7 ohms hot! Ran fine when revved... because the alternator voltage came up just enough to charge the coil just enough to fire the plugs adequately. That ties in with your comment on measuring if the system voltage is adequate at idle.