Your actual swingarm has much larger holes for the chainguard. If you remove the chainguard and leave those holes open will those keep you up at night???
The hole isn't in the swingarm, it's in the brace.
Drilling a #30 or hole in the brace isn't going to weaken anything. I work with enough metals (all kinds of steels and aluminums) and deal with engineering issues daily here at the airplane factory. If I was "stressed" about drilling this little hole, believe me, I wouldn't have done it. There have been plenty of 150+ hp bikes in the past that didn't even have swing arm "braces" and they went 170+ mph just fine on the race tracks day in and day out. The brace is just that, a brace, and is suppose to help prevent the swingarm from any twisting under high torque situations.
If the brace does develope a stress fracture I'll be sure and let you guys know.......and then I'll put a bead of weld around the hole and continue on riding. Meanwhile the actual swingarm will still be doing it's job of holding the rear wheel on the bike just as it was intended to do.