Probably a ton. At the mRNA level you could easily detect 50-100 fold more expression by adding the CMV promoter. This effect might be less dramatic if the endogenous promoter is quite strong. However, in general, the CMV promoter has higher transcriptional output than endogenous promoters across multiple (but not all) cell types.

250 fold, for low expression genes. As mentioned before, depends on your gene promoter strength

If this is to assess in vitro, then the expression will be directly proportional to the available molecular quantity of the un oxidized amino acid, regardless of the numerical value of the promoters.