Math and English subscores do not matter when it comes to comparing ACT and SAT scores. There are no conversion charts yet for the current SAT, but for the old out-of-2400 SAT, the only relevant information used to convert ACT scores to SAT scores was the composite ACT score. According to these charts, a 30 composite score would've been equivalent to about a 2000 SAT score.
The only reasons you'd want to have higher subscores in certain areas would be if you're applying to a specialized program (like engineering school) that might, for instance, value Math and Science scores more than Reading and English scores, or if you're applying to a school that superscores the ACT. Otherwise, colleges are going to look at your composite and section scores.