So you are using some c-code from another company to bit-bang an I2C bus master at 100-kHz and you want advice on how to speed this bit-bang routing up to 400-kHz? It seems like you would want to reduce some of the dumb waits in the bit-banger - the time that it sits and spins will driving the gpio hi or low.
FYI - the sexy fraternal twin of the CC2540, the CC2541, she has an I2C H/W peripheral where her brother has his USB. So if you switch to use the CC2541, you don't even need your bit-banging code from another company ;)