Its very noticeable online in a race such as NASCAR.
The other day i was in a long race on Daytona and was spun out so i decided to pit for new tyres, putting racing hard on for the long haul i was easily overtaking the 1st place battle without slipstream after being over a lap down.
Was in first place after maybe 1 or 2 laps as the extra power you seem to get just allowed me to breeze past them, of course they caught me with slipstream in the end