For competitive you are aways in a disadvantage if you are using the wrong nature for the job you want/need to do, also why both flare blitz and flametrower? Two look alike moves to serve the same purpose is no good in any situation.
However if this appease you, infernape can do incredible very well with modest nature, but to be the very best using this nature, he should fill offensive roles using special atk, let the physical moves and other roles aside to make the most of his nature. In this case you have to change your moveset.
Also for you luck again, if you gonna use your infernape with special moves for offense, his movepool in generation seven have many good options, allowing you to make the best of this nature with no problem.
Hope I helped, I wish you luck on your battles!