This is much more pronounced in the Japanese and Korean light cars. On a two laned road, when a B7R/B9R passes you from the opposite lane at high speed, you will feel the big shove from the air easily in these light cars. In fact even if you are parked by the highway and a big vehicle passes you at high speed, you will get a shove, it's normal. The effect is lesser in heavier cars and higher in lighter cars. Avoid overtaking long vehicles with a small gap, with a bigger gap the effect would be reduced. About your second question, it is always better to have the windows up at high speed as the resistance would be lesser, which in turn will aid fuel efficiency.