Oh, Negi's "10" the whole time, but he does get smarter and more mature. It just takes a while because he's so naive. He does a lot of growing up and maturing at an age where most of us have no idea what the hell's going on. I could give you a spoiler or two to entice you, but I don't know how you are with spoilers. Early on, I was letting it slide because FANSERVICE (aka PLOT as opposed to plot), but then he started maturing through his magic. He's also not as stupid as your typical shonen protagonist, or for that matter your typical harem guy.
Yeah, try it out. The Vampire of Sakura Lane mini-arc was only halfway done when you dropped out, and there's another arc that follows maybe two chapters after that ends and goes on until about chapter 53. With the characters introduced, the interpersonal relationships of the characters start to develop in preparation for the next arc, which gets into full swing around chapter 80, and spirals upward into the heavens from there, and that arc lasts 50-80 chapters, while the final major arc lasts from chapter 130-160 or so until chapter 317, I think. Then it's the wind-down chapters that leave room for an open-ended ending because Akamatsu quit writing due to both an expiring contract with Kodansha and the fact that he was trying to stop an ordinance from getting passed in Tokyo placing tighter restrictions on manga content, in particular the kind he's made a living out of writing (as they would be damaged by the blast radius of that ordinance, though not quite the intended core target).