It should take little time to understand that soft proofing is not to make the printer to print the colors shown on your monitor. It is the opposite. It makes the monitor to display the colors your printer will print. If you have a bad set of ink and you have a correct profile for it, or if you make a bad profile for a good set of ink soft proofing will make no improvement to the colors of the print. In either case the colors printed by the printer will be poor. Soft proofing will make your monitor to display the same poor colors the printer prints. No improvement is gained by soft proofing alone.
For example, I have a chrome silde of a Fuji Velvia RVP-50, which was the emulsion that dominated the images on National Geographic magazine in the 70's and probably early 80's. I may get a drum scan on Adobe RGB color space. The image will look fabulous on a high quality multimedia display with true fabulous colors. Now if I got this wimpy Canon Pro9000 with a set of cheap Chinese ink set it will not print the colors I see on my monitor, not even close. Even I use OEM ink it is not going to print the same colors seen on the monitor either.
So I make a profile for the cheap ink and I use soft proofing technique to make the monitor to display exactly the colors the printer will print with the cheap ink and the profile I made. The result is the colors the printer prints will be still the same, regardless the monitor displays differently before applying soft proofing or identically with soft proofing. Nothing is different. The only difference is the colors on the display will be close or identical to the print.
Chances are that the colors will be still different. Something is still wrong You will have a hack of good time trying to figure out why they are still different. The bottom line is if the ink is a set of bad ink or if the profile is poorly made for a good set of ink the colors in either case are supposed to be not great. Soft proofing will make it even worse if it is not done right and the colors between the monitor and the print don't match.
Will soft proofing make my printer print the fabulous colors on my slide? Soft proofing is not the answer to my quest of search for a solution to make my printer to print better. The answer is in finding a set of wide gamut ink and better tool to make a fabulous profile. Soft proofing does nothing to this.