Honestly, I always found Issei's progression a bit clunky. He gets his 'sacred gear'—that sentient gauntlet thing, Boosted Gear—right out the gate after being killed by a fallen angel on his first date. So his 'development' isn't about unlocking power so much as learning not to blow himself up with it. The whole 'doubling' ability starts simple, then he learns to transfer the boost to his allies, which is a neat tactical twist.
But the real growth spurt comes from his, uh, unique motivations. The dragon inside his gear gets stronger based on his 'desires,' and Issei's are... famously straightforward. So his power literally levels up because of his lecherous drive to build a harem. It's a weird feedback loop where his embarrassing personality fuels his battle strength. Later arcs add more gear forms and a balance-breaker mode, but the core mechanism always felt a bit silly, even for this series.
It's fun if you don't think about it too hard, I guess.