Having both the ST and a 916, I will say the OEM 916 mud guard fits pretty close to the tire where the OEM ST guard sits just a bit higher off the tire. Never really thought about it until a few years ago when I installed an actual set of sport touring tires on the ST. I'd previously used only sport tires like Pilot Powers. The sport touring tires had a lot more rubber and filled much more of that gap between the tire and mud guard. Didn't try it, but I bet the 916 mud guard would rub or outright not fit with ST tires.
That being said, both bikes now have carbon mud guards and both sit a little higher than the OEM parts. If you're going with an aftermarket carbon fiber part, probably not an issue.
No direct comparisons, but here are few few pics of what I am trying to illustrate.
For some reason only 3 pics are posting. Here's the ST with the carbon guard and sport tires. The touring tires take up more of the space.
Both carbon fenders are Ducati Performance parts, but definitely different years of manufacture. The one I got for the 916 was pretty old and needed a lot of TLC. The ST part was newer, probably around the same year as the bike.