Efficiency problems are usually caused by one of two things:
Bad crush. Not a problem in your case.
Poor volume measurement or not collecting all your wort. More likely.
With a very fine crush, a lot of flour is generated, so that in a “traditional cooler mash tun” setup, it can form a clay-like layer on top of the mash and significantly slow or stop flow of the liquid wort out of the mash tun. This is also known as the “stuck mash” or “stuck sparge” phenomenon. You need to take care to collect all the wort and don’t leave more than about a quart in the mash tun.
However, if you brew in a bag (BIAB), then this is more of a moot point because the grains are all squeezed to get all the goodness out, regardless of how much flour is in the grains.
Regardless of mash method, you need to nail your volume measurements. If you want 5 gallons after the boil, so that you collect 6 gallons before the boil (losing about a gallon due to evaporation during the boil), then if you are batch sparging, you want to collect close to 3 gallons from the first runnings and 3 gallons from second runnings. If you are fly sparging or BIAB, then just make sure you get 6 gallons or whatever you planned on getting for pre-boil exactly. If you plan on getting 6 gallons but accidentally collect 5.5 gallons or 7 gallons or something like that, this will screw up your efficiency calculations. You need to nail your volume measurements before the boil.
So… bottom line is, I don’t think a Corona mill can hurt your efficiency at all, if you collect all your wort. In fact, it should honestly HELP your efficiency to generate more flour, which is what I would expect from Corona. The only way it could hurt is if you don’t bother to collect all your wort, e.g., you intend to collect 6 gallons but give up with the stuck sparge after 5 gallons or whatever. That will hurt efficiency if you don’t collect all of your wort. Nail those volumes.