I have a set of Driven clip-ons. I put a lot of weight down on my bars doing canyons like Latigo downhill. I weigh about 225 so you can imagine the amount of pressure that rests on the clip-ons. I've never had any problems. I had a local shop do the work. I could have done it myself but I wanted to support the local motorcycle shop with 6 Ducatis in the showroom. Not sure if they drilled them or not, but the filing makes sense to me, or a partial drilling.
Here's some pics... hope that helps a little bit.