I had done one or two of those Bandito races, the only advantage a custom Bandito has is you can have 100% jump instead of 20% jump.
Now in the race I did everything was easily avoidable and you didn't need jump, I even won it in a stock Bandito, but if any races do actually require you to jump something, it should be easier in a custom Bandito over a stock one.
Otherwise there is no difference I can between them.
Like justinlynch3 said, the jump is the only advantage for racing. I did a few of the races, they are real crap IMO. Tarmac on all of them with very few tiny jumps. They need a track at the motoX track, one at the skate part by the beach and one in the hills somewhere. To start.
On the other hand, the PvP aspects of this tiny thing are incredible. Most people don't seem to know about it yet. When they see you roll up they are curious and walk right up to you as you detonate the C4.