To me it sounds like you're being givin the advice to gain more power by reducing the downhaul. In my opinion this is wrong. Yes you will have more power, but because of the bad aerodynamics and poor control you won't really achive anything by it.
I am pretty sure that most pro slalom sailors never mess with their downhaul. So why should you. Find a setting where you have good control and good power. And adjust the power of the sail with your outhaul.
Wavesailors, and freestylers adjust the dowhaul., but to my understanding, this is to gain more spin when looping and doing sin moves.
My own theory: You dont need to catch any wind to produce power. Think about it. if all the wind stays in the sail it would be like throwing a big balloon through the air. and if you have ever tried that you know the ballon wont go far. It creates huge amounts of turbulence behind it. that way actually sucking you backwards. But if you untie the ballon so that the air will be directed backwards(like the leach of the sail), it will go with huge speeds flying around the room.