It's noticable but I don't think it has been much researched or documented. Some of the books which describe the working properties of wood cite the moisture content, but not very specifically, neither do they compare properties at various moisture levels. This is perhaps the single most important consideration when building and using all wood bows, imho.
Also I think at the root of the contraversy over the years wrt to which species make the "best" bows. Lessons from antiquity, where performance was judged across a variety of conditions, casts a much different picture than the neo-dogma, which laudes the utility of most any species without as much regard for this consideration. Perhaps because, contrary to older times, given our modern lifestyles one can pretty much dictate and control the moisture content of one's bows without too much effort.
Unfortunately finishes and sealants are not very effective at combatting uptake of moisture. Wood "breathes" regardless of what you do. But some coatings are better than other, simple parafin wax for example can be very helpful. I use it on top of several coats of polyurethane. Modern catalyzed varnishes and epoxy finishes are also said to be more effective.