My flow is about 15x also. It's especially important for me to keep the Airstone running after lights out to help co2 off gassing happen quicker otherwise the tank stores it like a bottle overnight, this has knock on effects for the next injection period. Having the Airstone run all night my pH is at a stable high for a couple of hours before the gas comes back on. This stable pH point is my starting base point for the days injection (starting pH changes throughout the week due to tank influences, ie fish/plant waste & EI dosing). Running it 24/7 overcomes my filtration method of using a crook and not a spraybar to keep water/air contact high.
If your off gassing rate is already at a maximum due to filtration method and you need more off gassing then an Airstone can help, you can then bleed air from the line to fine tune the amount you need without having to fiddle with the needle valve on the co2 regulator (mines set a good rate and balanced by the air off gassing co2)
My tank is a small cube so the surface area is not great, so my margins for error are very narrow, and small adjustments have a greater effect. I think the larger the tank the less of a problem this tends to be.
If I had the space then I would be having a tank with an overflow and a wet dry sump so there is always a maximum of water air contact ensuring good oxygenation and maximally efficient co2 off gassing. I've read (TheBarrReport) that some users using this filtration method can off gas co2 within an hour of solenoid closing from a 1 point pH drop (mine takes 8 hours with the help of an Airstone). The canister filter represents a closed loop in the system ensuring that any gas transfer out of the system always happens through the tank surface area, which is effectively the closed neck of a bottle, the smaller the neck the tougher it is to off gas co2.
I hope that makes sense.
Other things I'm doing slightly differently is that I have augmented my air supply to the Airstone by sending it from two different pumps. One runs 24/7 with a line from it that I can bleed to adjust the bubble rate, the other is plumbed into the same line after a one way valve so that it doesn't back pressure into the other pumps line, this pump is controlled by an UP pH controller (CO2Art) set to LO, this way when the pH drops to a point where the fish will show signs of stress due to the amount of co2 going in but not coming out, it triggers the supplemental air to come on and blow of a little more co2 to keep the fish comfortable, I never have to reach for the needle valve, just balance it with air injection. The only thing I have to do to maintain this is to note my start pH for the day, calculate the drop and adjust the controller to come on just past this.
If the controllers could learn the daily pH stable start point and adjust their variances accordingly it would be entirely hands free, but alas these don't exist yet, at least not as a consumer item (not impossible though).