PD go to all the trouble to write a good physics model, & then they dumb it down with this nonsense by not making it optional...
👍 Bravo. That's exactly how I feel.
We praise and levy kudos on GT5 for it's physics model--arguably one of the best in the game console world. But then we have a slipstream effect which is nothing but a joke. A parody. You're right--it's a cartoon. And anybody who races in leagues where there's tight competition over 1~2 hours of racing, rather than 2~3 laps, will tell you that it completely destroys any 'proper' racing.
Take the PDI Kart and drive slightly behind another Kart and slightly off-set on a long straight, so that you're following behind but not in the other Kart's slipstream. When you both approach top speed, tuck in behind the other Kart and *WHAM!* it's like you've been hit with a giant sledgehammer as you suddenly, as if by magic, gain an additional 15 kmh in top speed in about 2 seconds.

In a Kart!!!
Or take any of the SuperGT (GT500) cars. With the right gearing and downforce settings, if you time it just right, you can gain as much as 35 mph (55 kmh) in top end by the time you're right behind them.

This is not how it works in the real world. And I'm just amazed that there are still people on this site who defend it and claim that it's realistic. It isn't. Not by a long shot.
In aerodynamically sensitive cars like NASCAR on an oval track, you can probably gain 5-6-7 mph in extreme cases. In this game--30+. It's ridiculous. PD knows it's not accurate. And it's there for one purpose. Because when it comes down to it, GT is a game. And it's a game that has to appeal to sweeping tides of casual gamers as well as hard core enthusiasts. And they're trying to shake things up and make it fun. The draft effect in GT5 is no different than turning on 'boost'. It allows less skilled players to keep pace with more skilled players. Not only is the slipstream much, MUCH too strong, but it's affects can be felt from 100m or more behind the car in front if you're going fast enough. Back in the real world, you would have to be much closer to gain any advantage.
I really could care less about single player mode. But there's already a toggle for "real" or "low" grip on the track in online mode. I wish they would include a toggle for the slipstream as well. Or if that's not possible, simply shut it off completely like it is in practice mode online.