Not "nearly horizontal" but angled between 15 to 30 degrees to the airflow. We're using them as stub wings to induce a delta swirl effect. It seems that this will create slightly tighter and more energetic vortices than just using them as near vertical fences. It also has the advantage that using them all in the same direction at an angle can direct the airflow somewhat to, give it a bit of a "kick" upwards at the rear, compensating for downwash effects. But I dunno if you'll measure a difference in your gaslog between using them like that or vertically, differences could be subtle, it just seems theoretically better.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice