I've used lots of McR20s cut down to 17mm in my E head mods.
As with machining any reflector, you run this risk of damage to the reflective surface, which can occur in varying degrees.
Although I do love the McR20, and the beam from the cut down McR17, bernie did a test for me and he got a higher lux reading from the same mod with an IMS 17mm reflector. I'm going to attribute this to the fact that the McR is textured and the IMS is not.
Since finding this out, I usually use the IMS reflectors in applications that allow their use, simply because I don't have to risk damage to the reflector while modding it.
I should also note that the difference between the two above mentioned reflectors was not big. I typically reserve using cut down McRs for "special" handmade lights, again, only because of the risk. If I could mod the McR with no risk of damage, I'd use them all of the time. The beam is simply beautiful.