How high up does your clutch grab on your s550? My 15 grabs ON the floor. If I don't have the pedal buried in the carpet, it will grind on shifts. As far as know, it has had nothing done to the clutch.
If it's on the floor it either has air in the lines or the clutch friction plate is worn. The master cylinder is inside the car above the clutch pedal. It should be completely dry. I've done atleast 10 that had a leaking slave cylinder and it would cause similar issues. Would get air inside that would make the pedal really low.
I'll check that out later. Where is the reservoir for the clutch master? I looked next to the brake master reservoir but didn't see anything. Or is it part of the brake master?
Update: I found this thread here, https://www.mustangecoboost.net/threads/urgent-clutch-line-loose.9289/ that showed clutch line fitment. I checked mine and sure enough it was leaking, which means it was probably sucking air. I pushed the line up and pumped the clutch about 150 times. This completely changed the clutch. It feels like I'm driving a different car. Gear changes are much easier. It used to be near impossible to get it in first before completely stopping. Also, cold shift crunch is gone.
I guess my synchros have been working overtime... oops.