Here are some comments I located on the "Atkins all the Way" site about Arise 5000 and 8000. The comments are from one of the employees from Honeyville:

"The WPI that we sell is Arise 8000 which is essentially Vital Wheat Gluten on steroids that makes it about 94 percent protein. The other type of WPI is Arise 5000, which have had their protein chains chemically cut to allow for machinability in commercial applications.

In the case of Arise 5000, the protein chains have been chemically cut to allow the dough to be not as rubbery, making it easier for the mixing blades to work and not bind."

Like I mentioned, I have both and use both for different applications. I have made, for example, the 3 Minute Chocolate Cake - one containing Arise 8000 and almond flour, and another cake containing Arise 5000 and almond flour. The Arise 8000 cake was heavier and denser (chewier) than the Arise 5000 cake, which was much lighter and airier (closer to "cake").

I hope this information helps a little.