I find a power meter invaluable as a training tool, but never use it in actual races. Racing, TT's in particular have to be 100% on feel. It's your own little pain cave that you crawl into. Too many variables affect just what watts you can sustain in any given TT so targets mean nothing.
What matters is crossing the line completely spent. I've won individual TT's and few TTT state championships. Never once looked at my Garmin in those.