To reiterate, a properly installed shift kit will:
1 - Reduce/eliminate gear overlap
2 - Reduce internal heat
3 - Increase shift speed and feel (crisper)
Number 1 and 2 are actually pretty important. Eliminating the gear overlap will reduce the wear on the gear clutches (they no longer have to slip against each other) and in turn reduce the amount of heat produced.
This is great news because it is essentially less wear and tear on the transmission, and ideally improve longevity (heat is THE major automatic transmission killer). Overall the installation of a shift kit will lose the stock slushbox feeling and drop some time off your quarter mile run. Who isn’t interested in this?!
However not all transmission are created equal, and shift kits can have an adverse affect on such slushboxed. For example, aftermarket results indicate that installing a shift kit on a 4R70W automatic transmission (1996-2004 Mustangs) can reduce lifespan and cause premature failure due to the higher line pressures.
For this reason, it is important to research compatibility for any shift kit you may wish to install. If in doubt, ask the manufacturer. If you have a Mustang with a 4R70W, look into doing a Jmod, replacement valve body or even an electronic tuner. Reliability is said to be much better with any of those. Mustang owners of the Fox/SN95 era (1979-1995) needn’t worry. Their AOD transmissions work fine with an aftermarket shift kit.