Rockstar Games fixed a GTA Online bug in the worst way possible - by making a blanket change that had adverse consequences on things that worked properly.
When the Tuners update launched in GTA Online, players quickly figured out that the new modifications that were specific to the new car class had pretty wild effects. They would make the cars drift like crazy but they also made them much faster in a straight line. As a result, Growler and Calico were able to reach insane speeds, leaving even the batmobile in the dust.
While fun, many suspected this was a bug that would get fixed later on and they were right. The fix came with The Contract update, at which point the Tuners only got a small bump in top speed for using lowered stance and low-grip tires. However, this change had adverse effects on the Tuners in regular setups too.
Their acceleration seems to have suffered and one of the culprits is the lost of brake-boosting that the drifting enthusiasts loved to use. The negative changes are not clearly visible if you're driving in free roam because you will be at top speed a lot of the time, which hasn't been nerfed. Going into a race will showcase the nerfs to the fullest though as all the vehicles in the Tuners class suffer from worse lap time now.
The information came to light thanks to Broughy's extensive testing of the cars so it would be wise to check out his video that goes into detail about the change. It's embedded above.