I have a 2004 MS 400 with the twin Yanmars. I typically cruise at 8-9 knots, about 2000 RPM if my memory serves me (and it sometimes doesn't). I will run at 3000 RPM from time to time and I hit 16 or 17 kts or a little bit more depending on the current (hit 21 on the C&D Canal last summer with a following current). I don't think I have pushed it higher that 3000 as yet. At the higher speeds I have no vibration or excessive prop noise and, on the upper Chesapeake, the ride is quite comfortable unless we get a strong beam wind which does make the boat rock side to side a bit.
If you do some research, you will get lots of opinions, pro and con, about the 3126. I don't think I ever saw anyone complain about this Yanmar engine though, so, for me, I wanted the twins both for that reason and for maneuverability, as I was already used to twins on my other boat. Of course it means double everything, oil changes, props, gauges, transmissions, zincs etc. so there's that as a negative.
I bought the boat in September 2018 so my experience with the boat is somewhat limited, really only one full season.
Generally love everything about the boat. Like any boat, there are always things that need to be fixed or upgraded. What year are you looking at?