Recent advances in the photonics devices integration bring ONoC as a bridge future for communication media in the MPSoC domain. As ONoC can support Wavelength Division Multiplexing (WDM) technique, communications between cores can be improved through allocation of one or several wavelengths for each communication. However, WDM introduces wavelength crosstalk, requiring to increase the laser power to provide accurate communication between cores. Thus, for the designer, exploring this design space (execution time vs power consumption) is not an easy task due to a large number of wavelength allocation combinations. The contribution presented in this paper proposes to evaluate the two extreme bounds of this design space considering the different communication scenario. To address this problem, we model the wavelength allocation by two different objective functions to compute the bounds in terms of execution times. Furthermore, from an accurate model of crosstalk between the wavelengths, we compute the energy penalty for each communication scenario. The results presented in this paper highlight the execution time and energy consumption tradeoff, and the opportunity for communication optimisation thanks to an efficient use of WDM technique.