It is not always a performance positive. It has been my experience that most tube dampers actually hurt the sound, albeit subtly. They seem like such a good idea, too. Experimentation is the order of the day.
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If the dampers impair the tube's ability to get rid of heat then you can expect a shortened service life out of the tube. But usually they are a good thing sonically.
I have heard people say that there is a problem with loss of air due to the tubes being overdamped. Such a thing is not actually possible, FWIW.