Framer - in general terms, the Boeing is built for simplicity. IF you maintain Vref+5 at any flap setting you will be ABOVE V2 for the g/a flap for that config (F15 or F1). The margin is generally positive (F15/30 and 40) except at very low weights where it can be negative, hence the 'teaching' of Vref+5 to be safe.
Certainly the 800 has about 4kts added to Vref F30 compared to the 700. I guess the 900 will be even greater but I do not have figures..