has anyone here had any experience tuning compression horns? with pieces of foam or placing material in the horn mouth, i understand the pieces of foam are to ajust the soundstage width! does it make a inprovement if done right?

While I never attempted this technique myself, I did use compression horns in my own car for a fair duration and have heard of it being used in the States. The design of the mouth of the horn itself is supposed to offer excellent sideways dispersion, but the 'vanes' are said to enhance this further. Why not simply try it out with some cardboard and masking tape? If it works then try a more permanent solution.

USD used to supply all their waveguides with small rectangles of stiff yellow fibreglass insulation material with which to tune the dispersion. I think I still have some from my original pair 10 years ago!
I would try it with the insulation as it is a porous material and will not encourage reflections within the horn throat like cardboard etc.