I have recently (May) set up a 120 gal (4x2x2) with wet-dry. I am struggling to keep my pH at 7.0, and to do this I have to stream my CO2 into the intake of my submersible pump at a huge rate!
I have ordered a plantguild CO2 power reactor, and I know that will make some difference over not using any reactor in my system.
But I discovered an interesting thing by accident: I did my usual water change, but over filled my tank such that the bioballs in the sump were almost completely covered. In other words, I am using a 20 gal tank for my sump, and the water level in the sump was about 3 inches from the top. I left it this way. The interesting thing is that later that day (maybe 8 hours later), the pH was down to 6.5!!! Nothing else was different in my setup. Therefore I deduced that the water cascading through the bioballs is causing a tremendous out-gassing of CO2.
The main thing I like about the wet-dry is the surface skimming, otherwise, I would have started with canisters to begin with. I may switch to canisters yet, but will wait and put the CO2 reactor on, see how the pH goes, before making a big investment and switch filters.
I may try changing my sump media around. Instead of a rectangular holding pen with bioballs, and the water dripping through a drip tray through them, I am brainstorming about using a cylindrical column holding pen, maybe 8-10 inches in diameter. Then I would have the water first flow through "filter fluff" (standard polyester stuff) as the first filter media in the cylinder as a prefilter. this would be above water line. Then the water would flow down through some permanently submerged filter material, similar to what happens in a canister filter.