you cant mesh mod the gen x pump on a aquaC and get better performance. it goes against how the AQUAC skimmer works. It uses spray injection and need the pressure provided by the pump.
Ph drop is not just co2. It has to do with the chemical makeup of the water. Your alk, mag, calcium, ammonia, phosphates ect... The co2 can make a difference but there is quite a bit to it.
as far as your skimmer goes. AquaC skimmers in my experience do work well. However compared to some of the new skimmers they are not as efficient. They require a large pump and are a little noisy. They are well built but do require a larger pump to run them just like a beckett skimmer.
If you are happy with the skimmer then great. if it isnt broke dont fix it, right?
Otherwise I would look for something a little more efficient. depending on cost I would look at the atb 1050A, the msx 250A the super reef octo's or the vertex in 280.
Thats a pretty wide price range. is there a way to put a fan in the cabinet that would help some.