Alex, the wooden body of the PD is an insulator. By insulating the migration of heat away from the Heat Exchanger it helps maintain the temperature equilibrium of the HE. In some ways the wooden body also acts as a heat sink absorbing some of the heat.
We set our optimum diameter and height at an American 12 oz. Soda or Beer can. About 2-5/8" across and 4-5/8" high. With that as a non-varying standard, and other variables held steady, wood density becomes the controlling factor.
Wood density varies, from Balsa to Lignum Vitae and maybe others on the outside of those extremes that I'm not aware of. Woods also vary in density from one tree to another and one part of the tree to another... ie. wood of any species is not homogeneous. One piece of Cherry may be more dense than another, same for any other species.
Given those qualifications, I can make some generalizations, that may not hold true for every single piece of wood in a species. But, will work, as general guidelines, based on a lot of woodworking experience and a lot of PDs made with a lot of woods...
Lighter (weight) woods are more "air filled", ie. have more empty space. You can crush Balsa, you can dent Douglas Fir, you can ding Cherry, you can try to harm Lignum Vitae, but good luck. That sorta gives you an idea of the range of hardness associated with the density too, but hardness can be associated with other factors. ie. hardness doesn't equal density in all cases...
So back to insulation... Given that wood is wood is wood, it's all basically made of the same cellulose, packed tight or loose (yes, that's an over simplification). Tightly packed insulation is not as efficient as loose material. So in general lighter woods are better insulators, resulting in higher HE temps, to a point.
At some point though, you realize that Balsa isn't going to make a very durable unit, and Lignum Vitae isn't a good enough insulator. Goldilocks syndrome. One's too soft, one is too hard. Too light, too heavy.
You need something that's 'just right...', or nearly so. We have always believed this meant Hardwoods. Softwoods as a category are out of the question. We have never used them.
Hardwoods then present the 'too dense' problem. The coolest temperature vapes I have ever made were Macasar Ebony and Pink Ivory. "Those were the days" as they say. Pammy still uses those prototypes for aromatherapy, but we consider them too dense and cool for other uses. You get vapor but it's not satisfying and wasn't something we wanted for our customers.
We also wanted to know exactly where our woods come from and don't want to promote 'poaching' of exotic native species, or the exploitation of native peoples. Thus, North American Hardwoods are our primary focus. American Cherry and Black Walnut from the Appalachian Mts. and Oregon Myrtle. All Kiln Dried (KD).
We have since found a very good supplier of exotics that has very strict standards.
The FSC (Forest Stewardship Council :
http://www.fsc.org/ ) logo next to a buyer for our supplier of exotics.
In the Beginning of 2008 I needed a 'house' wood. American Cherry was it, proto types were developed on a borrowed lathe in Myrtle and Western Maple
I wanted a consistent source of Kiln Dried lumber, I had worked with a lot of Cherry, it's easily available in the dimensions I needed. And just a bit lighter weight than Myrtle. Black Walnut is a bit harder than Cherry but about the same density on average.
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Got orders through 5-05 out (except a couple that missed the boat...) so hitting about 5 weeks in most cases (this week), but that can go from 4-6 weeks as we predict. Life can throw you a curve or two, or you might get clear sailing so we do our best and whittle away at it.
OO that inline tube sure is clever
, always a favorite shape NYC. Daddy was a scientist, after all, so . . . 
! The wait has been MUCH harder than I expected, but I'm sure it'll be worth it.
Tom said when he got a chance to he would answer your question. 
so I'm not sure how many if any emails I'll be able to answer today. :/ Hope they get it up and running smooth soon.