My comment was inspired by my memory of the Steingraber phoenix w/carbon fiber board at the 2011 KC convention. The air conditioning system was playing havoc with all the pianos. However, in the entire hall, the only piano that was seriously out of tune was the Steingraber.
Obviously there could be many more mundane explanations of this, such as Steingraber's tech was AWOL or awash in the lounge. One would assume the sales people who brought such a unique instrument before a high profile gathering of way-picky-conservative piano nerds, nerds who would be looking for flaws, would be paying attention to this sort of thing.
It leads me to wonder why the ambient reactivity was so strong, given the stability of the carbon fiber, lack of humidity driven migrating bridge pins, etc.
Perhaps Larry or someone with a more intimate day-to-day relationship to this creature could comment on its relative stability in normal and/or fluctuating ambient conditions.
As a whole, I find the Steingraeber Phoenix to have more tuning stability.
Tuning stability is always a collection of circumstances that must include how the piano was tuned and prepared. Additionally, material wise, the strings themselves are very sensitive to temperature. An air conditioner unit cycling, blowing cold air on a piano is killing any hope of tuning stability. The same goes for stage lighting. Anyone truly experienced in concert preparation know these things well.
I have found the carbon fiber itself sensitive to temperature. I can not say that by itself, it is more or less sensitive than it's wood counterpart. That would require a proper scientific process that I have not done.
I can say that, overall this Steingraeber Phoenix demonstrates a noticeable degree of tuning stability over it's wood counterpart.