I think with the 19" front, you might have different results than the older models with the 18" front.
Basically, as far as I can tell, they instructed 18psi in the front of the C5, to slow down the responsiveness of the handling in the front, so it didn't get too twitchy.
Eventually, they abandoned the 18psi band aid, and issued a 19" front with different forks, like you have. You will probably have better results with the actual recommended pressures for that tire.
That tire is supposed to have 28psi in it, not 18psi.