Running mostly 26 4 blades.
But I have a set of Mirage 27's/new to play with, for some top end wheels.
The boat is a straight bottom 42 with a modest X dim.
I want to get all I can with these and need about 400 rpm more to reach peak power, are these do-able?

I got some labbed 28 bravos. Or you could lab finish your 26s

Absolutely the 27's are a great choice for top end with the boat hull you are running. We need to know more about the current performance with the 26's. If the numbers are there, the 27's would be the go to propellers for speed.

Great, Then I won't waste time with any 25's.