If you useing assumed temp/flex you do not need to go to full power to mack it. You may well feel its a good move and I wouldn't disagree but the numbers assume you leave the thrust levers were they are.
If you've used a fixed derate (ie go into the FMC and derate the engines from 22k to 20k of thrust) and then flex on top of that then I think you can run into snags with VMCG if you go to max chat. I'm not 100% on that as we don't do it but dim and distant and all that.
While the discussion started off around the 80kt call there is no reason why the Capt might not fall over with a heart rending plea for help just prior to V1 so its as well to think about the high speed case too.