hi jes50 !
You 've got three pedals on your g 25 , and the speed system is sequential so you got a possibility to try ...
other wise you can set the boost , on something else than a bouton ...
i had the same difficulty with the nitro when i started to play that game , unfortunatly i got only the momo from logitech (only two pedals)
but i have found this compromise :
i have set the boost on my accelerator pedal but notice that you when you set it the game ask you too press fooly The command then to say ok and then the game ask you
too realease the command at that moment the tip is to realese only a little beat you command and so you will actionate the boost only if you ground the accelerator !
this my own solution and for the moment i 'm really enjoying piloting so

and i did set the air brake on the brake command becose the air brakes work only if you are in the air so braking is the same pedal on the ground and in the air (the game setting alow to put two command on the same one )
hope that can help you to find your own setting

, and welcome !

just excuse my bad english i'm french an eating too much frogs legs !

and remember no pain ! ....