To cut out the crappy grills, I use a dremel, then a "bastard file" to file down the sharp edges a bit.
For regular case blowholes I use a hole saw with a large drill press, then file it down.
For those of you that don't know, a "bastard file" is about 10" long and 1/2" to 3/4" wide and meant for filing metal. They are commonly used for sharpening archery broadheads. 🙂
If you think you'll be doing more case modding, you probably will want to pick up a dremel. Sometimes you can find them for pretty decent prices. You won't need a whole kit, just the dremel and some cutting discs. Mine isn't even adjustable. Only one speed.