It will be interesting if you can do a speed comparison back to back too with the 15.
My old Zodiac 340 Fastroller with Tohatsu 9.8 used to hit 17.5 kts on a good day 1 1/2 up with minor gear. The Aerotec (due to it's more draggy much deeper V 'hull' compared to the Zodiac's 'skaty' flat hull as I understand it) is a bit more demanding of power and needs the Mariner 15 to match this. It's touched 18.5 but usually tops out at 17 ish with the same load.
You'll need a 20mm raiser block on the Aerotec with the Mariner 15.