david weaver
In a panel plane that i assembled and one where I left only about 5 degrees of relief on the front of the mouth, I used this design - the shallow primary bevel and the very steep microbevel - with good effect.

It is a common pitch plane that previously had a (came with, at least) very rounded chipbreaker that couldn't have been used no matter the relief of the mouth because it was so steep.

I have limited options with the mouth, I could open it up another 5 degrees or so, but not more, because of interference from the bun when filing, but so far it's a trick that works great.

I have set the mouth (in making it) at just less than a hundredth and at common pitch in a heavy cut, it could still tear out (but not a lot). This kind of tearout is not necessarily problematic, but it does keep you from finishing the job with the panel plane, which is sometimes a nice thing to do.

At any rate, I'd suspect a lot of sloppy castings on common planes will not provide enough relief, just like that, to allow the shavings to feed. And this trick worked well on my panel plane to get it into a shape now where it will provide a very nice finished surface with a fairly heavy shaving, a virtue in the common furniture type stuff I have been building (I don't use a power planer).

However, if the mouth is already accurately set at 4 thousandths of an inch or so, someone will probably not need a tight set of the second iron to begin with.

the one page that included instruction (japan woodworker) said something along the lines of having a tiny steep bevel and a shallow primary bevel following it to break the chip, but then otherwise be out of the way so that the flow of the shaving is not impeded further. That sounds good, I guess, it helps with feeding more than planing force, I'd think - the work is all being done on the steep bevel of the second iron. Maybe I'm wrong.

I'm just happy that all of this has allowed me to step up my panel plane a notch without any further taping off of the bun and filing more relief.