What the chasm said. Some people climb very well OTS, others don't. A lot of that is body weight. Thin climbers go well OTS. I've ridden with one guy who was always faster OTS. If a rider is light enough, it probably comes down to specificity - one does well what one does most. At 5'6" and 158 lbs., I'm always faster ITS, all the way down to 35 rpm. OTOH, my legs get tired only doing one thing, so I'll stand from time to time, even though it spikes my HR. Everyone's different about that. Carmichael was always telling Lance to get his butt in the saddle, but he did pretty well standing too. If you're light and have a huge VO2max, you can do whatever.

You can train seated efficiency by doing some 50 cadence grinding intervals, or like you say, just climbing really steep hills seated. Don't move your upper body or allow the bike to change speed during the pedal stroke. Similarly you can train standing efficiency by say, doing similar 500' vertical climbing intervals standing.

My only standing tip to add the chasm's is to keep your knees close to the top tube.