The OP is right about the AI though, they're programmed with faster karts. They accelerate faster on straightaways and their top speed isn't limited. Even if you set your transmission dummy slider all the way down, they'll out-accelerate you on a straight. This is balanced out by the last 2 laps of every race, where the AI will slow down and you can easily catch up. There's some pretty significant rubber-banding going on too. If you screw up royally and spin out on an early lap, you can still easily catch up in the last lap.
The only event I had problems with was the 125cc Shifter Race #3 at GT Arena. I'm going for a perfect win ratio on the stats page, so whenever I didn't get 1st place in that race, I had to start over from the beginning and do over the first 2 races. Took me all day to finally get gold in all 3 races successively. The 1st two races become pretty easy once you get the hang of the kart and the courses, but GT Arena requires you to get perfect runs on every lap. I figured out that you need to average at least ~26.5 seconds per lap in order to win. I also experimented by setting the transmission slider at 86mph and also at 112mph, but it made very little difference and I still ended up with identical lap times. Here's a video of someone getting gold on all the Shifter races:
The key for GT Arena is to accelerate hard and brake hard on the first two sharp turns, then throttle your way into the last section. In this last section, follow a wide driving line and throttle starting at about 70%, gradually increasing the pressure until you're at the straightaway where you should now be fully holding down the gas. You need to overtake the 2nd place racer by at least the 7th lap. By the 8th lap, you'll need to overtake 1st place in any of the turns. Once you overtake 1st place, you control the pace of the AI and you can drive comfortably until you win. If you're right behind 1st place on lap 8 and you know you don't have enough time to overtake him, try accelerating as hard as you can out of any of the sharp turns and ramming him from behind. This will mess with the AI and cause him to slow down. I actually had to resort to this in order to finally get gold. Turning on the driving line and setting transmission to automatic also helps you focus on the race.
The Red Bull kart races are a cinch once you master the 125cc shifter races, they handle like a dream and you have to go out of your way to make them spin out.