Like many things in karting, it depends....
are you able to do quick testing laps: a testing lap is one where you can do each lap within 1/10th of a second or less, normally about 80% of your full capability, ie you slow down slightly to drive more accurately
a quick testing lap is one which although keeping your lap times within 1/10th, would also put you in the pack (for 'regular drivers' not front-runners).
If you cannot do quick testing laps, then you probably aren't going to see any benefit from 25mm stubs, because you are not driving precisely enough to see the benefit.
If you can do quick testing laps, then they are worth trying to see if they work for you.
The theoretical benefits are: they are often lighter than 17mm stubs because the actual axle is hollow whereas the 17mm is always solid The axle itself is stiffer than a 17mm axle and therefore allows a greater range of adjustment, where you would need to get a 17mm stub with an increased extension (the solid bit between the kingpost mounting and the axle part). The 25 mm bearing is meant to have less lock-up in a turn. (if you don't run an internal separator, you know about lockup, because you can actually see and feel the bearing acting as a brake when you do the axle nut up too tight. In a Turn the pressure on the wheel acts to load teh bearings in the same way, forcing the contact onto the side of the race).
I suspect that most of us run with 25mm bearings because 'the kart came with them' rather than because we have tested and found a difference or we are following suit, because we assume that the fastest drivers are using 25mm stubs because they have tested.
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