He's Sensei when we're addressing him and Jim Sensei when we're talking about him publicly (and it's anybody's game what we call him when he's not looking). We always kinda giggle when someone says "Klar Sensei." It just doesn't feel like they're talking about the same person. And, you know, it sounds funny.
I am never in a bind when it comes to calling Jim something; it is a familiar pattern to me and i know what to expect.
On the other hand, I often run into trouble when i meet dojo-cho from abroad and they introduce themselves be their first names. I was at a seminar this year and met this really nice guy named Charley, who later on I realized everyone else was calling "Sensei." I kinda didn't know what was better - i was not sure if he was chill with me calling him by name since that's how he introduced himself, or if he expected me to get the hint from everyone else. He was mad down to earth so it didn't really end up mattering too much, but it made me think. I guess most folks aren't too uptight about it.