We report on the investigation of four-wave mixing (FWM) in a long (1.3 mm) dispersion-engineered Gallium Indium Phosphide (GaInP) photonic crystal (PhC) waveguide. A comparison with a non-engineered design is made with respect to measured FWM efficiency maps. A striking different response is observed, in terms of dependence on the pump wavelength and the spectral detuning. The benefits and the limitations of both structures are discussed, in particular the trade-off between slow-light enhancement of the FWM efficiency and the conversion bandwidth. The time-resolved parametric conversion of short pulses at 10 GHz is also shown. Finally, the transmission capability of a 40 Gbit/s RZ signal is assessed through bit-error rate measurements, revealing error-free operation with only 1dB penalty.