[fuzzypuppy]

You will not get smoother shifts w/a shift kit but firmer quicker shifting, a shift kit reprograms your valvebody to change shift points as well.
you can gain a slight edge in performance but my main reason for the addition was to make my very expensive trans last a little longer with less clutch slippage with shifting which keeps the trans cooler and less cluch material floating around etc. though with more than 100k on your trans you might be looking at trouble installing a kit, with that many miles on the clutch's and bands they may not take to well to the quicker firmer shifts.
I wouldn't rebuild a trans without the addition of a shift kit though.