In the past, there have been doubts about whether Xiao Feng has reached a similar level of martial arts as Zhang Sanfeng. Zhang Sanfeng was famed for his Single-Stance Execution abilities. One such instance was when he sneaked up on the hands-occupied Xuanming Elder and could proceed to finish him in ONE stance. Now, I have found a similar evidence about Xiao Feng's Single-Stance Execution ability in DGSD, when he could have finished Grandpa Tan as his heart desired.
Zhang Sanfeng's description:
原来张三丰悄没声的欺近身来,左手已轻轻搭在他的肩头上。那人大吃一惊,心知张三丰只须内劲一吐,自己不死 也得重伤,
Even if Xuanming Elder would not die, he would suffer severe injuries.
Qiao Feng's description:
谭公手足微微一抖,这时他兀自被乔峰提着,身子凌空,乔峰只须掌心内力一吐,立时便送了他的性 命。
If Qiao Feng would have unleashed any palm internal energy at all, Grandpa Tan would surely suffer immediate death.
Qiao Feng had been exchanging several blows with Grandpa Tan before taking complete control of him. But let's not forget that Zhang Sanfeng never took Xuanming Elder head-on. Zhang Sanfeng sneaked up on the Xuanming Elder while the latter's hand was occupied. If Xiao Feng would have sneaked up on Grandpa Tan, I'm sure he could have taken full control of the latter's body, considering that Xiao Feng even grabbed Murong Fu's entire body under his mercy in one simple stance when the latter was occupied.
Now, it says that Zhang Sanfeng may or may not be able to kill the Xuanming Elder instantly, whereas Xiao Feng would definitely kill Grandpa Tan instantly. Neither Zhang Sanfeng nor Xiao Feng at this point in the story was at their peak yet. Zhang would spend 10 more years inventing the Taiji art, while the novel explicitly stated that Xiao Feng had improved a few months later.
One consideration would be whether Pre-10 years Xuanming Elder is on the same level as Grandpa Tan. My opinion is that Xuanming Elder would have slightly superior but the difference would not be very big. Grandpa Tan had launched a triple-palmed LDA which neutralized one of Xiao Feng's palms, that says a lot about his martial arts.