To use a 30" shaft motor you will have to raise the transom of the existing bracket 5" or get a bracket setup to use a 30" shaft motor. You will bury the motor if you try to use a 30" shaft on 25" setup bracket and won't achieve anything by doing so.
Why are you wanting to go with a 30" shaft? The motor should be well above the water on the bracket if it is mounted correctly.