When you enter a race, if your 'Ghost' opponent's car is tuned, you will see a badge appended to your opponents details before the race, which says 'Tuned'.
Of course, if you make a mistake, it's going to affect your run and your opponent will most likely swish past you like you're looking for parking, because their speed hasn't dropped (assuming they didn't make a mistake at the same time). So it's reasonable to expect your opponent to fly past you if you make a mistake.
What's totally unreasonable, is if you are racing a 'stock' CCX for example and you have a maxed out CCX, with a perfect start and a clean run, there should be no way your opponent could whistle past you if you're driving well, because their PR is the same or less than yours. Of course, if you drive badly, then your opponent will overtake you.
I've witnessed so called 'stock' opponents overtake me in a straight line when I'm at Nitro max speed on a perfect run. That shouldn't be possible.