I note that Hatzell if memory serves is working on a five blade conversion for the -400.....rumor has it that it will kick the airplane up by 50kts.
I'm gonna have to call BS
on that one. I assume that we are talking KIAS Vmo. The airframe is the limiting factor there with respect to Vmo so don't expect the Cheyenne 400 to get any faster. But...a new prop can increase efficiency so that higher cruise speeds can be realized.
Of course nobody cruises at the ASL Vmo figures so chances are that a new prop design can add some speed, but not to that Vmo figure which looks like an ISA value.