It's unfortunate, although great, that you're massaging the fit of these parts. I'm following this like a wolf on the trail. Not sure if you've seen this, adventures in finding neutral:
http://www.xs650.com/threads/finding-neutral-while-stopped-and-in-gear.47568/
From that, I'm pretty convinced that the elusive neutral issue comes mostly from the complicated articulations of pulling that 4th slider outta the 1st's slots. Now, we want that thing to stay in there during 1st gear launches, but would be nice to be able to easily pull it outta there in spite of the minor clutch drag.
What I've sensed is that the shift actuation components easily enter "bind" when the effort to retract that 4th slider is increased. Hence my suggestion to polish the thrust side of the dogs and slots.
And, now that I think of it, especially with your reporting on the tightness of your new 4th slider, I'd like to advocate polishing the thrust surfaces (sides) of the shaft's splines and 4th slider's splines.
I've figured that there would be about 2 ft-lbs of torque being applied to the splines, slider, dogs, and slots, during the 1st-to-neutral shift. If you held the shaft, with the 4th slider fitted on the splines, and applied that 2 ft-lbs torque in the thrust direction, feel for the amount of drag while sliding it back-and-forth on the shaft. The goal would be to reduce, minimize, that sliding friction.
The only fin damper thread I know of:
http://www.xs650.com/threads/fin-vibration-absorbers-for-a-xs-1.47937/