It's partially true, a "dead" feeling string bed is all about how you perceive the feel to be. If you are stringing your syn guts very tight, you might even like the feel of a (looser!) poly. Since polys are stiffer in nature, you can drop the tension (some recommend up to 10%) and retain a lot of the control. However, you should be aware that the stiffness can cause joint pain (elbow, wrist, etc).
As far as the tension loss, yes, polys tend to lose tension fast regardless of the technology, some polys are much better than others, but none are stand out IMHO. If you are looking for more info, feel free to create a separate thread