No such thing as over kill. the 30C will net around 150amps continous which is over the recommended 125amps from HPI and Castle. While the 45C will give you major breathing room at 225amps! This is also important especially if your pushing 6S Keep in mind, that the larger the breathing room for your amp draw, the less harder it is for the battery keep up with the current load. That will extend the life of your batteries as well as reduce stress on your ESC, which has been the major cause of ESC melting, batteries that can't keep up with the current load from the ESC = smoke and sad feelings

For lipo batteries and ESC's, theres no such thing has having "too much" rated discharge