Shift kits.. do you really need one????

I used to do transmissions for a living in another life :-) and I can explain a little.
First, the cup plug is not for applying but for timing the release of the previous gear, so that you don't end up with a flare between gears.
I don't have a shift kit in mine because I can do what I wanted with internal mods.

I have the pressure raised and the separator plate drilled for quicker firmer shifts and a spring on my TP rod to delay the shift a little.
Under normal every day driving my car shifts into second at about 20-25 mph, and into 3rd at about 40-45
I don't need it to bark tires on the shifts, just a little quicker apply.
Doing it this way does not raise the wide open shift points, only the low and midrange shift.
If I also wanted it to shift later at WOT I could take some weight off the governor valve to do that.

The main job of a shift kit is to get into the next gear faster and with less slippage, and at a desired RPM.
Wanting quicker shifting is a no brainer, and less slippage of the clutches on the apply means less wear on clutch and band material and less heat buildup in the trans.
This can be taken too far though to the point of premature hard part failure of shaft splines, planetary gears and sun gear shell teeth. (overly hard shifts)
In my oppinion, the most imortant parts the shift kit has are the springs to make the shift valves operate at the right times, because when you raise the mainline pressure it affects the timing of everything else.
Kick down (passing gear), for example will change a lot from a relatively small change in mainline pressure and the shift kit spring compensates for that higher line pressure.

A person that knows what they are doing can do almost eveything a shiftkit can do with a drill, grinder and a few little springs, but most people are not capable of doing it.