GMAT Strategy

I am actually a GMAT trainer and am having a debate with a soon-to-be online educational company as to the most effective teaching approach (pedagogy) to the GMAT (or any standardized aptitude test, for that matter). I believe that a rigorous, systematic, step-by-step method that, by definition, limits understanding and creativity of thought is the most effective way to go. For example, I believe in limiting Sentence Correction to 7 basic rules and drilling these same rules over and over and over and over until my students are on automatic pilot by test day (by the way, the most important rule: make the answer parallel to the non-underlined portion). The educational company believes in taking a far more comprehensive approach: teaching every grammar rule the GMAT might test and providing multiple explanations to each question. Naturally, a mixture of the two approaches would be ideal, but I'm wondering about expert opinions as to which approach is more important (for example, I do go over the 12 rules of verb tense, but I don't emphasize them). If you could provide any hard evidence, that would be great. I also figure that this discussion might be helpful for students prepping for the GMAT. Thanks