Although you wouldn't think there would be strong mineral deposits with a water softener in line but the most frequent cause for lowered water pressure on a shower head is mineral and salt deposits. here's and easy fix if that is the problem:
Pour white vinegar into a plastic bag until itÂs half full. Pull the bag over the showerhead until its spray channels are submerged. Tape the bag to the showerhead pipe with electrical tape and leave on overnight. Scrub away any remaining buildup with an old toothbrush.
Hope this helps.

What does the vinegar do to a satin or brushed nickel shower head?

I only have experience with vinegar on chrome but i googled for effects of vinegar on different finishes and found several references that it is safe on chrome, stainless steel, fiberglass, etc. I dont see why it would be a problem on a nickel finish. I also found that it is highly recommended as an effective deodorizer. You learn something new every day.

Do you like your Grohe Thermostatic Shower Valve? I realize that there is a pressure issue now but have you otherwise been happy with it. We are thinking of doing this valve with the body sprays too.

I have one also but just since Nov and love it. When I would be taking a shower before and some one was using water in another part of the house, the pressure would go way down and the tempature too (some).

At this point, it seems pretty certain that the water softener is not functioning correctly. I can bypass it and notice a change in pressure.

However, I had contacted Grohe about the problem and got an email back from them saying that the "screens may need to be cleaned". This was the first I had heard of any maintenance associated with them.

As far as the thermo valve itself, love it. One of things that makes you wonder how you ever did without.