The best benchmark estimate for how strong Igneel was before Acnologia stole his soul is Ignia, given that he is Igneel's biological son. We know for sure that Ignia is stronger than Mercphobia because a single attack from Ignia had enough power to make Natsu strong enough to beat Mercphobia. Mercphobia actually tried to put out the flames with his floods, but the flames left over were still enough to make Natsu stronger than Mercphobia. The attack also didn't fully destroy the city of Elmina, which was well within Ignia's capabilities as he was known for destroying countries. Even accounting for the fact that Mercphobia was at half-power, this level of dominance is just too much. Ignia is not even personally fighting as a dragon, with the physical strength, claws, and teeth associated as one. If Igneel was at this level, then he's probably stronger than Mercphobia.