I think it's great, one of the "wave one" no brainer choices alongside the halfling and the tilean marksman. Sometimes I even hire him from the begging with my starting warband, depending wich band I'm running. But I think he isn't overpowered for his cost, 50gc and 25gc unkeep seems way too much, he would become a "wave two" choice and I'd rather prefer a much better supporting assassin or an ogre meatshield then. Pitfighter is great from the beggining, and may become a beast with 2 or 3 advances, but he's a hechmen and can't add more than 1 to his initial stats (starts with maximun S and T, so no advances there) and is pretty limited earning exp compared to heroes. If you hire him from the start, he starts to fall behind heroes after 4 or 5 games, when his unkeep bring his cumulated price to 90-105gc (that's vampire/possessed price range). If you hire him at that moment of the campaign, he's still great for 30gc, but not so powerful compared to the biggest threats on the table.
Pit fighter is powerful when he charges and he's supported by other guys (he's even good all alone against multiple weaklings), but with one wound and no special rules he's vulnerable to charges and ranged... and this guy is a crossbow fire magnet. I think you may see him as overpowered for that reason, as I recall from past messages your group doesn't use ranged at all.
Think about measure/counter-measure to balance the game with tactics rather than houseruling what isn't broken. Your rival hires a powerful close combat guy with no ranged at all, feel free to hire a good marksmen and place him in exposed on a great spot to take that guy down and threaten the whole band as the enemy has no way to counter attack him. Then your rival should hire another good marksmen as counter-measure or make a runner hero to take him down... and it goes on and on like that.
I think you have a great example here in this Reikland vs Withc Hunters BatRep. How Reikland crossbows win the game by delaying the most dangerous foes, the Flagellants. Only one of them was taken OOA, but the most important thing is crossbows made the most dangerous foes irrelevant during the game:
Sorry for the long post... vacation you know haha.