For the most part, my air control is working great. The draw seems to get tight at times even if I haven't made any adjustments but usually returns to normal (strange). For me the adjustable range is perfect...from airy to fully closed, and everything in between.
I don't think opening the intake slot will increase airflow because the airflow is also limited by the hole size of the connector pin...which appears to be larger in diameter than Ody and Penelope. If your range isn't there, try a larger oring on the control ring.
The only improvements I can think of is; a larger oring at the base to prevent leaking, eliminate (or notch) the "stopper ring" on the mouthpiece. (it get's in the way of refilling through the top cap)
That's it!