Same question I have asked the shop supplier in England and got their reply as below :
"Glad you like your sidecars.
The speed control that comes with the kit will only cope with motors down to 23t like the one supplied.
In our experience it is counter productive to put faster motors in the sidecars. They are hard enough to control as standard.
Brushless motor and speeed controller would be ok but only MILD, equivalent to 22t.
Best Regards
Chris
Formby Models"
Yes ! this is good idea to use this size for battery, I'm using the same. By the way have you installed the above optional item V2BR Ball Race Converter ? as the kit came without bearings for the front and side wheels.