You probably already know this, but just in case: The speedometer on your boat is using GPS. Upside? Dead-on land speed values displayed. Downside? Water current isn't accounted for in the reading (like it is with a paddle wheel).
I learned two things from your last post:
1) There's a page missing from my shop manual (the first page of the 210 Challenger information)
2) There is a "max RPM" listed on the page I didn't have - 7300 +/- 50 - just like your year; and, a rev limiter setting (from the page I pasted earlier)
I'm going to check the max rpm's on the engines next time I power it up and let you know what mine say. I'd be interested to know what the max RPMs record show for your engines. It can be accessed on the display menu.
Regardless, I don't have an answer for you - just more intrigued than ever.