I am not sure if igneel did match acknologia... Surely he performed well but in the end acknologia seemed to have a somewhat overwhelming victory. If igneel was at any point much stronger than acknologia then I don't see how the events would have unfolded as they did. Why not just defeat him 400 years ago? Instead the result was that all the dragons today, including igneel, were already half killed by acknologia. If acknologia was able to kill all the dragons that were within the dragon slayers 400 years ago then to me it would seem like even now acknologia did not really show all of his power against igneel.