If you have your diffuser right next to the output. That isn't enough time to dissolve. I think all you have to do to avoid the 7up look is extent the output tubing after the diffuser.
Another thing diffuser has going for it is size. I don't mind a little less efficiency if I don't have to have a bazooka under my cabinet. Plenty of reports to the contrary anyway. Lot's of people claim to have reduced their co2 input after switching to one of these inline diffusers.