A while ago I read what someone thought of that diffuser and they didn't like it at all. It was eventually replaced it with a Boyu glass/ceramic diffuser and he reported back to us with 30ppm CO2.
If you want a low tech solution that's what I would do, go with a diffuser. Drs. Foster & Smith also sells an Eheim diffuser that many are happy with. Turtlehead is selling the Boyu in the classifieds section. These have been showing up in plant oriented LFS around here, so check around, this is where I got one. Of course there is ADA's line at www.aquariumdesigngroup.com.
I was just curious because I'd never heard of a membrane diffuser before. For the moment I'm using the bubble ladder from my old Nutrafin Co2 System with my new pressurized setup on a 30 gallon tank. I'm still experimenting but 1bps gets me to 30ppm in a few hours.
That is a fairly inneficient method of using pressurized supply (but a Hagen Ladder is always useful to have on hand in case of breakdowns). Why not run the CO2 into your filter intake (unless you are using a HOB systems). If you have an Eheim filter, it will not lock up with CO2 bubbling into the intake. You should be able to get a similar CO2 level with about 40bpm by that method.