Honestly, I wouldn't mess with the suspension, other than getting it back to stock capability. Caveat being if your sister is really planning to drift it, since it's not nearly stiff enough for that. But, for learning RWD+MT, I don't think it gets more benign yet responsive than a Miata.

ps. lower + stiffer doesn't equal "rides nice for long highway trips" on a car with maybe 4" of suspension travel.