The most recent study (2006) [2] , performed by researchers at Leiden University, tested a Volcano Vaporizer with preparations of pure THC and found that:

ā?? Our results show that with the Volcano a safe and effective cannabinoid delivery system seems to be available to patients. The final pulmonal uptake of THC is comparable to the smoking of cannabis, while avoiding the respiratory disadvantages of smoking. ā?

When using plant material (crude flower tops), besides THC, several other cannabinoids as well as a range of other plant components including terpenoids were detected in the plant material. However, using pure THC in the Volcano, no degradation products (delta-8-THC (D8-THC), cannabinol (CBN), or unknown compounds) were detected by HPLC analysis. Also, a substantially larger fraction of the THC was delivered to the vapor by using pure THC.

Analysis of the vapor from the Volcano found that it delivered 36% - 61% of the THC in the sample [1] - a more recent study using pure cannabinoid preparations achieved a maximum of 54%. [2] For comparison, studies of cannabis cigarettes smoked via a smoking machine under varying conditions of puff duration and air speed found very similar efficiencies of 34% to 61%. Consequently, users can achieve the desired effect with a similar amount of material as when smoking.

In a 2001 study testing a device called the M1 VolatizerĀ® [3], the researchers found that "it is possible to vaporize medically active THC by heating marijuana to a temperature short of the point of combustion, thereby eliminating or substantially reducing harmful smoke toxins that are normally present in marijuana smoke." The M1 Volatizer, produced THC at a temperature of 365 degrees Fahrenheit (185 degrees Celsius), while completely eliminating three measured combustion products, benzene, toluene and naphthalene. Carbon monoxide and smoke tars were also reduced, but not quantified.

Please have a look at these results. It states that the analysis of the vapor from the Volcano found that it delivered 36% - 61% of the THC in the sample [1] - a more recent study using pure cannabinoid preparations achieved a maximum of 54%.
I know the Vaporstar extracts more THC than the Volcano because I've extracted THC vapour from the leftovers of the volcano with a vaporstar, hence the vaporstar is more efficient than the volcano.
And if someone isn't prepared to backup what they say, how can I believe them?
Just by looking at the leftovers from both the difference can be seen, from the Volcano weed comes out a deep Khaki color, after the Vaporstar finished it's deep brown, cooked.