I am sure Tom knows more than I do. On some of my low tech tanks, water changes don't seem to make a difference, for good or for bad. However, on some of my tanks, more frequent water changes lead to algae outbreaks. They types of algae are always ones that are said to be cause by inconsistent CO2 levels. I always assumed that the added CO2 gets used up quickly since it's not usually available in much quantity which gives a situation where you quickly raise the CO2, only to quickly bottom it out, which seems to cause algae.
From a practical matter, my last 3 low tech tanks got either minor or major algae outbreaks. Usually some form of BGA. This is an algae I have never been able to get rid of without using antibiotics. Stopping water changes was effective in all 3 to get rid of it. I tried the same on a high tech tank, totally not the case, had the opposite reaction, BGA just got out of control.
As per injecting less than 30 PPM, I would guess you would reap the same benefits as injecting more, just with less growth, less need to add ferts, and likely less water changes. A few people do an essentially low-medium light tank that doesn't need CO2 to be algae free and inject less and have seen benefits. It's usually because they have expensive fish and want a margin of error. I have never personally done it because I either want low tech, or high tech, the cost of injecting CO2 into a tank that doesn't need it isn't appealing to me. Other members have been successful.