In the last episode of the coaching video of Zhao Jianhua and Xiao Jie, they talked about the tension of string that they are using. Zhao Jianhua uses 28lb and Xiao Jie uses 22lb. I remember one thread mentioned about Zhao's reason of using 28lb is the sound of hitting. But I think what was not told was that he said he likes the 'direct' feel of power and control. Xiao seemed not to agreeing with him as she only uses 22lb. I think the take home message is that, regardless of the tension of the string, you can still play good and skillful badminton. I have seen people are getting obsessed with using some ultra high tension (>25lb) regardless of whether it suits their level or style of play. Some may just want to showoff how 'advance' they are by string at 30lb. In fact, any racket that can go beyond (manufacturer recommended) 25lb was rare of around 6-7 years ago, with the newer materials and manufacturing technology, rackets (and strings) can withstand higher string tension whist remaining light. So, think twice before stringing your racket at high tension, it may not do you any good.
String tension is personal preference, just as the racquet you use, and even down to the type of string, grip, shoes you wear. As you OP just mentioned, ZJH and XJ did not agree, because they prefer different things. Some people require more control over the shuttle which only these higher tensions can offer, its not about power. Knowledgable players know, for power you string lower (lower tension being all relative to the player's natural strength & technique of course) In my case... When making our proper strokes, we cant always perform a hairpin tumble at the net, with 25lbs consistently because the shuttle bounces 2-3 feet up, leaving us open for kills. We also do not want to take the shuttle later all the time, if we get there earlier we get more options in how to handle the shuttle. Yes that may be my lack of skills in that department, but I like to minimize margin of error as much as possible from having unpredictable bounce from the strings, hence having tighter strings; which allows me to focus on stroke technique further. Again, that's my preference, and it helps me play my game better. So you did partially hit the nail on the head, that no matter what tension you play at, you can play well. What is best for me, may not be best for you is what the 2 Master's are implying.
btw what is the type of their string? different string have different feel and playability at certain tension