I did it last year with black rain gutter mesh from Home Depot, and I was very happy with the results (I don't have the greatest pic at the moment). It's basicly a $5.00 (CDN) Mod. I think it called to use a hacksaw to cut the grille, but it might be easier with a rotary tool.
Here's the DIY on how to do it....
Civic Mesh Grille DIY

On the SI/SIR Thing, I think it looks a lot cleaner without emblems in the grille. But that's just my opinion.

Good Luck.