Lack of engine upgrades for me on GTA Online.

I'm pretty happy with my record, 23 wins out of 63 races and I'm getting more consistent as I go on.

The most important thing is when another racer does catch you (which is inevitable, the slipstream/catch up is so OP), just let them past. Swing way over to the other side of the road if you have to do, don't let them barge you off a cliff or something. Let them past and then either fly past them yourself or if you're sure you can do it without wrecking yourself, PIT manouevre or knock them off the road.

Learn how to take corners properly. The car handling in this game is very unrealistic, so you don't have to think about it like you would in Forza or Gran Turismo, you can attack every single corner hard and fast. Still though, visualize your line before each turn and get familiar with how whatever car you like best handles under braking.

Also stay calm and never give up any race. I've won several races from last place that seemed impossible. If the first 2/3 get into a real dogfight, they'll practically be at a standstill, they could easily lose a 30 seconds to a minute on a normal point-to-point. Not getting involved in shoving matches will save you bundles of time.

I wouldn't worry too much about performance upgrades, when I win I feel it's more down to my tactics and how well I've kept my head throughout the race. A standard Entity is fine for any super. I do generally race in a modded Bravado for muscle races, but it's not too heavily modded. I'm kind of undecided about what car to use for sports races, I do have a modded Entity but I think I over-did it with the turbo.

Motorbike races are complete lottery

Have to say I think the adder is such a useless car for racing. Loads of people use it just because it's meant to be the veyron, but I honestly think it's the worst performing super car in every way for racing. Heavy turning and slow to reach top speed.