It will be a very impressive piece of tuning, software and/or chemical engineering to increase maximum output by 20 hp without altering the hardware of the PU. Although that is yet to be comfirmed. Hopefully the knowledge they now have and the new limits that they set themself will have an effect on all Strat settings and bump power up a step compared to the old settings. Red Bull Honda do need more race pace too, it's not like the end of last season where Red Bull Renault was relativly stronger in the race than in qualifying.
Regardless, qualifying in a better position is always helpfull. Secondly Horner seem to alude to another (actually quite obvious) advantage:
“But we see after the pit stops, both Ferrari and Mercedes, it looks like they go back into a qualifying setting, particularly Sebastian [Vettel]. His one lap pace showed he did a 34.8s and then suddenly he is in the 36s. There is one standout lap.”
More power even if just for one lap will increase the chance to pull off an effective undercut.