I've heard of a few people with scores that improved trying this technique, but most students I've talked to didn't see any change. The problem with this technique is that a few extra minutes does not usually help you get the main idea. It also assumes you won't make any careless mistakes on the passages you do focus on. If your main issue is rushing which leads to silly mistakes, then it might help. Otherwise, I would try looking to the questions first to see if that helps you focus on what is important. Always read for the main idea and don't pay much attention to the details.