In NTSC, the Long Boosts study hasnt been made yet. But it seems that yea the other characters dont show any benefits , so we keep playing with bowser/dk as they have the fastest normal speed.
In PAL, El Nico (registered in the PAL version of the GP site) , has made
some studies about the Long boosts.
And it appeared that we could have Super Long boosts , with some special timing and with special characters.
Basically mario/luigi and koopa/toad are the characters that can have the fastest/longest Long Boosts. And as mario/luigi have a fastest normal speed than koopa/toad , we prefer to use them (as you can see with the pal champion records , florent lecoanet).
Of course its an advantage to use them only for course where there are a lot of Long boosts.
If the course is harder , then we can prefer koopa/toad also
But basically Mario/luigi are the best.
Only bowser/dk stay the best where there arent a lot of Long boosts and if the course is not too hard.
To get super long boosts , you have to trigger the shroom (or take an arrow) when your speed is in the red area.
Its also better to be quite far from the wall before jumping and hitting it (then releasing gas and keep pressing R) if you want to have a good long boost.
You need to have your speed increasing when you do that.
And also hitting direction while shrooming may damage the Long boost. So its better to push direction again only after you hit the wall.