I would expect that as library is called BGRA that you will never use different pixel format like RGBA.
On one side, we want speed and on the other side, we want same pixel format. Those wishes are contradictory.
It is possible to use BGRA order anyway by changing compiler directives (in package options, section "other"). Simply add the following:
dBGRABITMAP_BGRAPIXEL
The only thing that one need to be careful about is when writing/reading directly TBGRAPixel records, or if you access components using PByte. One can call SwapRedBlue before and after saving/loading the data if the value of TBGRAPixel_RGBAOrder is different from what is wanted. Otherwise what can happen is that red and blue colors are swapped. If you use functions provided in BGRABitmap, this is taken into account. Unless I made a mistake of course.
So I would say that for most users, that don't use PByte or blockread/blockwrite image data, nothing changes.
It seems that this backend is not set automatically for any platform. It will be able to select this backend in LazPaint to test speed? Can OpenGL really help to LazPaint as bitmap editor? It can be used just as final drawing part or can speed up some image operations in future?
No no, it is not used by TBGRABitmap. It is an extension for OpenGL with objects names prefixed by "TBGL". One could imagine that the OpenGL version would provide some acceleration for some things, however that would only be with the packages like BGRABitmapPack4LCL_OpenGL or BGRABitmapPack4NoGui_OpenGL.