Always seemed that way to me too. I mean, no question I’m faster with a dropper but I’m also more relaxed, working less hard, and sometimes able to do overtakes and lines that I wouldn’t have with the seat up.
I’m not an XC racer and I know that races are by and large won on the pedals but if you arrive at the bottom rested and in good shape you can surely attack the next climb harder, and one easier pass on a descent or one crash avoided cancels out a lot of weight difference too. 350g or thereabouts of weight penalty isn’t nothing but it doesn’t seem so huge. (and that’s for a retrofit, consumer post, if it was built in the frame and built to race spec/intervals it could be less)