I actually did experiments climbing steep slopes seated and standing with the power meter, and, at least in my case, standing resulted in a noticeable hit to my sustainable power. To the degree where I could put out 250-280 W for the duration while sitting, but only 180-200 for the same period of time standing. To make matters worse, I can go 280 W sitting for 8 min and I'll be recovered fully within 10 or 20 minutes; but going 200 W standing for 8 min causes physical muscle pain that does not go away for the duration of the ride.

I have no doubt that this can be trained for, but, for an average Joe, climbing out of the saddle is a last-resort option whenever you're out of gears.