Yamcha may have lost the first rounds in the 21st Budokai, 22nd Budokai, and 23rd Budokai, and he may have been the first to get killed against Vegeta and Nappa (against a Saibaman at that), and the first to lose (and almost die at that) in the Android/Cell Arc, and he may be the only Human Z Fighter without an OP technique like Kienzan and Shin Kikoho, that allows him to keep up with the Saiyans and Piccolo, but I actually see Krillin, Tenshinhan, and Yamcha as all being in the same ballpark in strength, with Krillin being noticeably stronger than Tenshinhan and Yamcha due to the Super Kami Guru power up, and with Tenshinhan being only slightly stronger than Yamcha due to him having more potential as a warrior.
With that said, I think he could easily keep up with Krillin and Tenshinhan had they all kept training. They could never hope to catch up to either the Base Saiyans or Piccolo, but I could see them maxing out at Cyborg #17/Cyborg #18 level strength. I'm talking about in the Dragon Ball Manga of course, since I have no real knowledge of Super.