I drilled out my stem to put my custom Di2 head unit in it. I was going to use a Ritchey stem, but my head unit wouldn't fit in it, but it fits in the 3T one! If it fit in a Ritchey WCS that's what I would have used though.
I have been using this for about a year I think. No problems at all and lots of sprinting out of the saddle. Before I did it I consulted with one of the head engineers at Cervelo (I am a structural engineer) to get his take on it. We both felt that drilling a carbon stem was a bad idea, but that an alloy one would be OK because before you got close to a failure, you would see stress cracks develop on the stem. I check it regularly, but it still looks perfect. The holes in my stem are 1/4" diameter.