I managed to change baudrate, but it was on Raspbian. I think it could be similar on Ubuntu Mate...
You just have to add the line
options i2c_bcm2708 baudrate=10000
in the /etc/modprobe.d/i2c_bcm2708.conf file, if you want to set 10000 baud/sec.
I did this modification because I had some communication issues between an Arduino like (a TheAirBoard) and a raspberry pi 3 for large baudrates.