Maybe we are not thinking about the right test on this thing. GTR's would probably rev up to 6, 7 or 8 grand normally? What if you were to put your tach innards in a vice, hook a clutch cable up to it, and then power the clutch cable with something like an electric drill and see if those cups play nice together. This would assume that the clutch cable you are using is OK. If you had an old cable you could just use the inside drive part of it or make something that might work so you could take the cable part of it out of the equation.
If the cups are rubbing due to rust maybe you could put some abrasive in there like glass beads from a bead blaster or some very fine white sand and spin the thing for awhile with the above cable and drill thing and wear the rust out of there. A little compressed air would clear the abrasive out and then hit it with some brake cleaner to wash out the rest.
Like I said... I drink a lot