You will definitely need a pinion with less teeth to go from a 17.5 to an 11.5. Don't run the car with the kit pinion as you will very likely damage the motor, speedo or transmission - or all three!
As recommended above, ask at your local club about pinion selection for your new motor. If one of the faster drivers recommends a ratio, take one tooth off the pinion to start with to be on the safe side.
Run the car for about two minutes, then stop and put your thumb on the motor. If you can hold it there for two seconds then you are about right for temperature. If you can't fit a smaller pinion, if it's easy (the motor feels no more than hand-warm) then go up by one tooth.
The reason there is not definitive ratios for brushless motors is that it depends how you drive. If you have high corner speed and the motor does not lose a lot of revs at the apex, you can gear higher (more pinion teeth) because it is doing less work to accelerate the car. If you have low corner speeds or use the brakes a lot, you will need to gear lower (less pinion teeth) because the motor will be doing more work.
The problem with the motor doing more work is that it gets hotter and that will eventually cause the motor to fail. In doing so, it can take your speedo with it. Too high gearing will put lots of strain on the transmission and could break something there. HTH
