I have a Custom II that originally had an external extractor. It had occasional extraction issues. I called Kimber asking about this same thing, they said run 500 rounds through it (break in period) and let us know. So I did, called them back, and they had me ship it back to them. When it was returned to me 3 weeks later, new slide with an internal extractor. Guess what, still had extraction problems. I replaced the extractor with a Wilson bullet proof, and replaced the recoil and firing pin spring with Wolffs. Now, finally, it runs perfect.
The whole external extractor thing is potentially arguable. I like having a 1911 that is as close to the original design as possible, and that has universal parts. I know how to tune a 1911 extractor. I had no idea what to do with the external one.