I'd like to see this 'book' on RNP.
They may have "wrote the book" or "cooked the book"...its a fine line, as I am not sure to whom you refer. There are many of us who have fixed that book.
Your post combined classics and NG...so I am wary of your context.
Fly a 900 at 3 degrees at Vref+10 and manage that energy on AT..(AT?)
On RNP, the auto throttle is engaged, so for one to 'fly' at Vref+10, one implies the AT has been degraded.
Another option to attain Vref+10 would be use CAT D for RNP on the 737...
So, many options or avenues...
lets open that 'book'