About ramjet efficiency in supersonic speed. Is there an optimal speed?
Well, that would depend on the design parameters, but for example this ideal ramjet would, with stoichiometric combustion process,  provide best thrust per unit mass flow at a speed of approx. 2.4 Mach


  ...the best operating range of a hydrocarbon-fueled ramjet is $ 2 \leq M_0 \leq 4$ . The parameters used are $ \tau_\textrm{max}
= 10$ , $ a_0 \approx 300\textrm{ m/s}$ in the stratosphere, $ f_\textrm{stoich} = 0.067$ for hydrocarbons, such that $ a_0/g
f_\textrm{stoich}
\approx 450\textrm{ s}$ .