Believe both. There is no conflict with your latest person honestly saying that 2 to 4 hairs per minute in thermolysis is what you can expect when working with her, and this board saying that 800 hairs per hour is possible from some practitioners, on some clients, depending on where and what is being worked on, and what equipment the practitioner is using, and how that practitioner utilizes her or his equipment.
Unless you find someone who at least is utilizing their auto-sensor, (and the auto-sensor being used has set capabilities of less than one second) you are not likely to find 800 per hour in your area. Furthermore, it is also less likely that such a speed can be reached when the hairs are spread out widely, and or, all growing in different directions, or the treatment area is hard to reach (such as under the chin of a large breasted woman). There are many variables here, and even those who can reach 800 hairs per hour, may not be able to give that to every client in every situation.
As for your other question, blend would be a better bet, if your hairs are curly, or have any distortion, as one would tend to have less chance of blowouts, and temporary pigmentation. Don’t get me wrong, it is perfectly possible to do work in thermolysis even with the curly distorted hairs. It is, however, easier to learn to work those hairs in blend, than it is to learn to work them in thermolysis, and it is here that the differences in machines and equipment start to separate thermolysis operators.
If your practitioner knows how to do both, you can trust her to do blend when that is the best thing to do, and thermolysis when that is the faster, but just as effective method.