I have the Satos for the m900 and they require the pressure switch.
Mind you, I don't use one since I never use my rear brake, and I've never found myself in a situation where I both needed my rear brake and I needed to let folks behind me know I was braking.
But it still bugs me I don't have one.
I use my rear brake 90% of the time. It makes for much smoother stopping and when needed I can stop on a dime using both.
As far as the pressure switch goes. Get the rearsets you want. If they require a switch buy one, they are pennies compared to the actual rearsets.