Official translations are out. And low and behold the things I've been saying were mistranslations for months turned out to be mistranslations. First off Suzaku questions if Natsu was holding back, not if he himself had gotten worse. Next Natsu doesn't say he got significantly stronger than the last time they fought. He agrees that there's no way he could grow so much stronger in a short time because you
accumulate strength by training daily. He then goes on to explain that his
accumulation is the experience he gets from facing someone before and learning about their movements. Natsu in the labyrinth isn't any stronger than in elentir and the scales he used for defense were not DF scales. It's literally just FDK flames that he's been had (and this was the same in the fan translation so I'm not sure how people got this wrong)