Kimimaro was a lot stronger than some people think, or maybe it's just me who underestimated the healthy Kimimaro. The first hint I noticed was Sakon's speech about Sasuke who apperantly didn't have nearly as much potential as Kimimaro. The second and more significant hint was told by Kidoumaru, his comparison revealed he wasn't as close to death since Kimimaro the day Neji sealed his fate with the final Jyuuken hit. Given this information I guess what happened most likely is the Sound 4 started an attack on Kimimaro to test his power and Kimimaro beat all of them nearly to death, probably interrupted by Kabuto or Orochimaru. Uchiha Sasuke couldn't control his cursed seal when the Sound 4 were playing with him, so Kimimaro probably didn't have an advanced cursed seal either yet he still have fought the Sound 4 and won or at least beat the shit out of Kidoumaru, a foe that even the genius Neji thought was impossible to defeat 1on1. The Sound 4 were afraid of Kimimaro, Tayuya was scared when she meet him again on her mission, yet they were not afraid of two enemy Konoha Jounin. I surmise as a healthy kid Kimimaro was at least kage level. In his last fight with Naruto, Lee, and Gaara he was with one foot in the grave yet still show insane skills.