Jie Feng (correct spelling: Xie Feng), the Beggar's Union chief in the Smiling Proud Wanderer era --> how good is his martial arts? Judging from the following 2 (brief) implications plus a platform-wide observation, I think his martial arts may be on a similar level as Abbot Fangzheng and Ren Woxing.
Evidence #1: Jie Feng can write on wood with his finger as profoundly as Abbot Fangzheng.
Evidence #2: In chapter 34, Yue Buqun said: "My martial arts cannot compare to the likes of Abbot Fengzheng, Priest Chongxu, and Chief Jie Feng." By lumping Jie Feng with the 2 superpowers of the era, Yue Buqun was suggesting that Jie Feng is on a similar level as those 2.
Now, my personal observation is that the Beggar's Union Chief in most eras is on a similar level as the Shaolin abbot (except when the chief is a leading character). In pre-DGSD, Wang Jiantong should be on par with Abbot Xuanci, seeing how they both led the assassination of Xiao Yuanshan. In ROCH, Lu Youjiao is probably as bad as the Shaolin abbot. In HSDS, Shi Huolong should be near the level of Cheng Kun, who is near the level of Abbot Kongwen. In Ode to Gallantry, nothing about the Beggar's Union is known. Only in LOCH and present-DGSD, when the chiefs are central characters, are they more powerful than Shaolin abbot. Since Jie Feng only made one appearance in the novel, by association he belongs to the list of chieves whose martial arts are on par with the Shaolin abbot.