I should note that it is not quite obvious that the blocked area is the only statistic of importance. Typically, it may be better to have a few big blockages adding up to 15% than a lot of small ones adding up to the same area.
Brain-Storm sells two high-flow grills: one looks like the usual plate-with-holes sort of grill, except that the material between the holes has been reduced to almost nothing. The other one consists of a number of parallel metal strips, allowing the air to flow between the strips. This is the grill I bought. It looks bettter. Although the spacings between the strips are much larger than the tiny holes in the other kind of grill, nothing big (pieces of tire, exhausts, big stones, etcetera) will get through. And even small stones are likely to be slowed down.
I did not notice any adverse effect of the grill on engine performance. So why was I still not satisfied? Well, the metal strips that form the grill are just that, metal strips. In other words, they present a thin rectangular cross section to the flow. Aerodynamically, such a cross section is known to be quite inefficient since it introduces large scale separation right at the front.
As a fluid-dynamicist, I know that for almost two-dimensional bodies such as metal strips, better is possible. You can reduce the effect of the strip on the flow to almost nothing by giving the cross-section of the strip an aerodynamic shape instead of a rectangle. Typically, the best shape is like an airfoil (wing): a rounded front and tapering off to a point at the rear.
Although I could not notice adverse effects due to my grill, that does not mean there were none. And making the strips more aerodynamic will certainly not hurt.
So I first took the grill apart. (It is simply screwed together.) Then I took up a file and started rounding the front edges of the strips! Am I mad? There are 8 strips, and the longest ones took me 50 minutes a piece! Well, at least I do no longer have to worry about whether or not I lose performance due to my grill.
To round the front edges of the strips, I first filed down the 90 degree corners under a 45 degree angle. This already brings you somewhat closer to a smooth front edge. Then I filed the front edge to a smooth path for the air to bend around. Air flow does not like to bend around sharp corners. After that, I was really not up to filing the entire strip into a taper from the front to a point at the rear edge. And the most critical area is the front edge in the first place. So I merely filed the 90 degree corners at the rear edge slightly down under about a 45 degree angle. This should improve things a bit more, and you will never get the air flow to follow the contour all the way down to the middle of the rear edge in the first place. Finally I used a finer file to improve the smoothness of the surface and painted the grill to improve the surface smoothness even more.