I reckon for the same power on a proper TT bike with the right geometry you are looking at 5kmh instantly when you get down in a good aero position !I instantly go from 35kmh to 41 kmh by getting down there.
A road bike on the drops may get you low but you must consider the 2 big pillars called your arms that are going to be either side of your torso causing massive amounts of drag as opposed to mostly in front of your body breaking up the air that your torso is about to go through.
In a pack on a road bike this is no big deal but when it's just you it makes a massive difference.
You also have to consider the effects of a road bikes geometry on the run .
Tri bikes place you in a position that uses more quad and less hamstring .
You are sure going to need your hams for running !