Meliodas' ultimate power level has not been officially released by the author.
The last time Meliodas' power level was estimated, it reached a total of 142 000 (Volume 29). This was Assault Mode Meliodas, which is theoretically his original form and strength (as described by Merlin and other characters).
However, after his possession by the Demon King, Meliodas indeed increased his power by an unknown amount. Most notably, Meliodas' power is considered so huge that he was decided to to leave Britannia, in order to avoid destroying the country through his sheer presence, which warped the climate / weather by itself. Assault Mode Meliodas did not have such a natural effects on his surroundings.