I cant speak for street bike’s on the track but I can for SX/MX. My KTM SX bike was a few pounds heavier overall than my past bikes, yet was more nimble, quicker suspension and just was night and day different than the last bikes. I bought it from a pro rider who moved away from the NE and got new bikes every year. It was hopped up pretty good over the factory 250SXF.
You can tell the differences riding between a few ounces of mud stuck to the rims, swingarm, fork guards, etc. The suspension “packs out” in whoops, becasue it cant rebound quick enough before it hits the next bump. Also lighter weight wheels make a difference in the air too. Panic reving to bring up the front end is slower, even mid air slight adjustments from reving or breaking to change the pitch are noticeable.
I have been on plenty of other riders bikes at the local tracks. Stock VS modded same model bikes are night and day different. I would LOVE to get a chance to ride a factory bike. Those bikes have thousands of dollars in stupid shit to save weight. Titanium hollow axle shafts that save thenths of an ounce… stupid small difference, but in the big picture for a rider that is on a bike more than you and I infront of a computer screen are invaluable.