Hey,
The Gamebuino is based on Arduino Uno, so it uses it's default 100kHz.
They explain in this Arduino forum post
how to increase it to 400kHz, but I doubt it's really necessary.
The advantage of I2C is that you can have many slaves with no extra pins thank to the adresses system. But if you want more speed, you can use the SPI bus (talking about MEGA hertz here, wow!) which is accessible though the 6 pin ISP port on the left of the Gamebuino. Beware, as this bus is shared between the screen and the micro SD, don't leave your slave enabled when not communicating, or it will interfere. You can use the pins from the I2C as outputs to drive the chip select pin of your slave.