I have no doubt, after doing quite a bit of experimentation, that polys will generally perform "better" at tensions below the recommended ranges (in the 40's). At these low numbers, there is considerably more power and spin produced for an equal amount of effort at higher tensions. And, for poly, lower tensions are definitely easier on the arm. There is also a tradeoff, IMO, as I believe the increase in power does come with the sacrifice of some control; not enough to be unacceptable, per se, but noticeable.
Still, the concept of playing polys at lower tension can be difficult to fully embrace. I am among those who regularly string a stiff, heavy frame around 60 lbs. I will normally do this even with poly or a poly hybrid. Racquets that I have known and loved with this setup have included the K90, KPS88, RDS001 Mid, and RDiS 100 Mid. The RDiS is my current stick.
In terms of my tennis, I am an unabashed control freak. I have never had any difficulty producing my own power, so --- perhaps predictably --- my emphasis has always been on accuracy. Polys at high tension enable me to think and execute shots in terms of precise placement. That is why I will indulge myself with polys, even though their playability rapidly diminishes with age. I find that control with polys at lower tension requires a delicate balance of power and spin. As I seek to employ more spin to harness the added power, it is easy to apply too much, resulting in a shorter ball. It takes time to find an acceptable compromise.
Higher tensions also provide me with a particular "feel" and audible cue that I rely on heavily during play. Appreciable loss of either factor can lead to a certain feeling of disorientation and resulting lack of confidence on shots. I have noted, in my experience, that lower tensions produce a commensurate lower pitch off the strings and a more muted feel. Thus, my first reaction when testing setups at lower tensions is an immediate sense that "this is all wrong" and "I don't like it". I have to really stay with the combination for a while to start to see the benefit and make a fairer assessment.
I have also found that the quality or style of play of my opponent matters. Poly setups at low tension seem to play better for me when facing an adversary who hits a penetrating, heavy ball. This makes the stringbed come alive for me. By contrast, playing someone who plays more spin, touch, or junk can be frustrating, as the lower tension offers me too little feedback and too much power. That's obviously not going to be true for everyone, but it is something that I experience as a relative newbie to the low tension setup.
I suspect that most people who are accustomed to using higher tensions in their frames would find suddenly switching to a lower tension to require a big adjustment in their play. Speaking only for myself, I find that higher tensions --- even with poly --- are not necessarily better, but are seemingly more natural, given long-established habit.
I will continue to play around with poly at lower tensions as I enjoy doing different things. I can also see the advantages of poly at low tension. However, for the time being, the majority of sticks in my bag will remain strung around 60 lbs, which is actually middle-of-the-road for most manufacturer's recommended tension ranges. YMMV
Cheers!