Width - do we go for a wide tyre for more grip or a slim tyre for less rolling resistance?
Pressure - do we go for low pressure for comfort or high for less RR?
My understanding is that good aerodynamics and low rolling resistance are essential. To achieve this , I am assuming that the thinner tyre with higher pressure would be the better choice.
Standard tyre pressures for the C5 aren't that high, I suspect this is due to the plastic rims. I run my steel front wheel at 50psi, and edge the rear ones up to 40psi.