I used to use them. They do degas A LOT. The biowheel design as well as the internal spraybars tend to degas a lot of CO2. Let me give you an idea. After the CO2 is off, with the Emperor 400, my ph went up to 7 pretty quickly within 3 hours. With my current setup (using only the API Rena canister filter), the ph only went from 6.3 to 6.6 in 3 to 4 hours. I don't think all HOBs can degas a lot. With the Emperor 400, there are too much water splashing going on (the spraybars and the moving wheels). Is it as bad as wet/dry filtration and with a sump??? I don't know.
Ive decided not to use the emperor, thanks tetra73!