Your view may be the wow factor- You may be better to put your wow into the splash back as ultimately it is less expensive to change. A total rebuild has many areas for cost over runs so with a large area, I would look at your overall budget and how much you are allocating for bench tops - This will help determine what you can afford and ultimately your choices. Depending on the surface dark benches can be harder to keep looking clean. I suggest you borrow some samples and bring them home - wet them and wipe them etc

I currently have the same problem It quite braves to use White on the floor ( I know you said grey) but we are toying with as it works well with the grey bath we have chosen Have a think about swapping it around to have black on the floor which works great with a contrasting colour bath (white) Also think about contrasting colour for taps (black taps against white or grey tiles)

How about doing the reverse for master: black Shaker cabinet (possibly floating) with a white stone benchtop? It would also be helpful to see size of master bathroom, ie sketch or plans. You could use style of handles similar to your kitchen, but in a satin chrome or nickel (since matt black wouldn't stand out so well on black cabinet). Have you thought about a basin? Couldn't be recessed, partly recessed or above bench. Will you have downlights w/pendant light or one or the other? Would be most helpful to see a pic of your kitchen too, since you want a similar flow and style, and that will help others help you.

With two tone Federation Kitchen Cabinets, we usually use the darker colour on the lower levels so as to avoid creating a sense of the room being top heavy, but having said this; your Kitchen should always be a reflection of your own personal taste and individuality. As with these images indicate, using a darker colour on your overhead cabinets can work when choosing the right colour and placement and so I encourage you to go with what you like.