Hi @yuki,
ArduSub sends its IMU data via SCALED_IMU2 MAVLink messages, and does not currently use SCALED_IMU by default (it’s listed as requestable, but I’m not sure what data it would have if you did request it). The SCALED_IMU2 message is sent as part of the SRn_RAW_SENSORS stream group, and the frequency is controlled by the corresponding SRn_RAW_SENS parameter.
Note that increasing the telemetry rates can cause bandwidth issues, and may overwhelm the code you’re using the parse the messages. The autopilot has access to much higher sensor data and output control rates (and with significantly lower latency), so if you’re doing this because you want some form of stabilisation control over the vehicle then it’s likely better to implement that as a flight mode in ArduSub rather than via an external program using MAVLink communication (assuming the feature(s) you want aren’t already available within ArduSub’s existing flight modes, of course).