Bit rate reduction when converting 60fps footage into 30fps?
In theory, if the number of keyframes and their placement were about the same, as decided by the encoder, then yes, you can reduce the bitrate to half and retain the same quality. In practice, the encoder may be configured to place a keyframe every N seconds, in which case, you should use a bitrate a bit higher than half the original bitrate, say, around 55%.