My C4 has a shift improver kit in it. Gives nice crisp shifts, not harsh and certainly not sluggish.
A mate of mine years ago had a more extreme shift kit fitted to a turbo 400 with a 350 chev in a panel van, it used to squeek the tyres on up shift from 1st to 2nd and for the rev head in us all :f used to be great fun.
All said though, it's a preference thing as to which kit you select. The shift improver kit does make the trans shift much cleaner, extends the life of clutches and bands and just feels nicer.
I did not want the chirping of the tyres on gear changes, so I just told my trans rebuilder that and he chose the particular kit fitted