I just wanted to thank everybody for the help here, especially MississippiMan for being so helpful with good advices
Till all those tests I have done in the last days I am really beginning to understand how important a darker than white projection material in projection generally is - be it under ambientlight conditions or a totally black theater room.
Because even when the walls do not reflect light back to the screen, there is always ambient light in the room, ALWAYS !
We ask ourself which light, if the curtains are closed, the lamps are off and the walls and ceiling are black
The answer is simple, the projector itself is the ambient light
When a projector shoots out light, this light is always present in the room and at the screen - and white material just can't absorb any of it.
And this fact will always degrade black levels and shadow detail and there is absolutely nothing we can do against this, accept to absorb light from the parts of the picture which should be black, and this can be done only with the screen itself.
The only time when a white screen in a dark room shows really good black levels is, when a high contrast projector with and added Autoiris shoots a completely black image on the screen.
If there is light in the picture, this will always wash out the black parts on the screen to a proportionally
higher extent than on a darker surface.
So a screen can really boost native contrast , if it preserves the contrast of the projector as good as possible.
Not On:Off contrast - BUT intra scene or Ansi contrast.
Only a while ago, I asked myself if a grey screen is really absolutely necessary, and I thought if I can make the room as dark as possible the idea of a darker screen becomes unnecessary for me.
But after a lot of thinking and comparing, I came to the conclusion that also the light from the projector will always wash out blacks on a white screen to some extent, no matter how dark the room is.
With a darker screen the intra scene contrast raises everytime the tint of it goes darker, in EVERY condition of the world.
The problem which remains is, that our eyes need enough light on the screen to feel that whites are really white and not grey.
Our brains know how white looks and if this is to dark and dim, then the raise in contrast is not enough to please our eyes.
If we would have silent and ecofriendly 10000 Lumen projectors and combine that to a black matte screen, even in dedicated home theaters we would have a big raise in perceived intrascene contrast and shadow detail because the screen would absorb every light from the black parts of the picture and this would preserve the complete Ansi Contrast of a projector, which is high enough with today's machines and reaches up to 1000:1 with High-End DLP projectors.
Even the Sony's have about 400:1 which is enough for our brains to think black is really black in mixed scenes.
What I want to say again is, that I am very happy to better understand now the value, which a darker screen has - even in a Batcave home theater.
For me personally, the RS-MaxxMudd LL v.2.5 paint till now is the best compromise between good whites and satisfying blacks in a dark room
The silver fire for me personally uses too much silver, which I personally find too annoying with whites.
But with high ambient light levels, you don't have much choices, I also understand this and for this purpose the silver fire was made for.
I only have to find the Rust-Oleum ingredients and will be happy for a long time with the RS-MaxxMudd LL v.2.5 paint I think.