Aero plays huge part. Along with karting I also do quite a bit of road cycling. Aerodynamics is a huge part of that industry and bikes only go 20-30mph. Now your power output is much lower than a kart but still aero is very important. They say anything over 15mph 80 percentage of your pedaling power is just going to penetrate through the air. So on a kart I would agree with Brian that anything over 55-60mph aero will play large roll.
On my laydown shifter I have made small changes to body work and found it has changed top speed by 4-5mph and changed lap times by 2-3 secs. Heck one time a gopro placed in a bad place cost 3 mph and 1 sec in lap time.
There was a Jay Leno garage episode were they ran a tag kart on a sprint track with and without body work and I don’t remember exactly but I believe they lost over a second on like a 40 sec lap.
I would except at Daytona in a yamaha laydown you would see a difference of 10 plus mph and 10 plus secs.