It wasn't THAT subtle, Yuuji's been working out since he got fucked by Pheles. But I wouldn't call his development steady by any means.
"steady" makes me think of Michishio Nagasumi. His improved strength was shown all the time but it was never commented on until the final episode - over the course of the series, we saw him get lucky in circumflexing Shark Fujishiro to being able to hold Gozaburo and Edomae off AT THE SAME TIME, walking circles around Kai and then beating the shit out of the mermaid prince fag.
Both Yuuji and Nagasumi are awesome but Nagasumi's GAR was the better developed. I know about novel Yuuji so I won't have to sing praises about him, but anime could have been this awesome longer.
I mean, in 2-3 episodes he goes Sairei no Hebi. We can't enjoy the manly very long.