Let's talk about "shift firmness" OK?
IF you are running a bone-stock 4L60, it should have what we called "boulevard shifts" that are somewhat early by about 300 RPM and a gentle application of the TCC.
From there as a base-unit, we go up in firmness and shift timing ..... and remember that shift timing is very critical and represents maybe 70% of complaints or concerns by the customers when they get the unit back.
If shift timing is late, it tends to be --- er ... harsher, for the lack of a better word.
The engine will be closer to - or in the Power Band at that time and if can send more ponies to the unit and that results in some old person's bag of apples from the market scattering all over the floor.
Old people don't like that --- well, SOME don't.
If shift timing is early --- they happen so softly and get into OD so fasgt that there's no feel at all of it happening.
We call that "stacking" the shifts .... and it's bad for many reasons .....
...........in that the front pump is being lazy and not producing much pressure or flow ...
...........and the clutches and band(s) ... remember we're not just talking about 4Ls here .... their friction material is gently pushed into contact and they kinda spin or slip a whole lot more than they should ....
...........and this will kill a transmission and waste a lot of fuel and can cause all sorts of other drivability problems.
Low pressure shifts are a problem in a different area too -------------------
The annular sealing rings won't be pushed out to their sealing surfaces, and this causes blowby of the apply fluid especially around the stators and shafts that are required to not only establish concentric motion of the spinning parts in relationship to the slower --- or faster --- glanulus (hollow) shafts that carry fluid commands and lubrication to critical parts.
Some annular rings are keylocked and some are scarf cut and some are solid needing an installation tool and a swage to reshape and reform them ....
Then again ... some are just o-rings and they need a little pressure to seal themselves up too.
I digress ........
If some shifts are early and some are correct --- this might be to clearances being too tight in one or two areas ....
....... or accumulators being stuck in their bores ....
........the bores can be ruined with gouged material missing, promoting bypass leaks ....
....... and all thereby failing to modulate/moderate and affect the timing of the shift ...
....... causing application overlap and premature friction material wear ...
........................all kinds-a problems.
Most "firmness" problems are therefor a perception and may not be the correct reality .....
..... whereas "softness" IS ---> in and of itself ---> suicidal in a transmission.
It causes a lot of heat and creates a lot of unnecessary wear in that one friction unit has not fully released .....
.......when along has come the next friction unit too early, albeit it softly ---
.......but they fight each other for a few moments ..............
.......and if the convertor doesn't have the capacity to "eat that difference up" ---
--------------> or absorb the mismatch application/release ......
The clunk then becomes a complaint and it's back to the shop --- again.
Once a customer has had a failure like any of those....
..... especially if it happens more than once ----
..... they are on the lookout for even the tiniest "something" that they feel now ---
..... and YOU have a very hard time selling the job is now correct.
They don't believe you.
Me, as an 'old guy' .... I LIKE a healthy shift ---- one that can chirp the tires if I wanna!
Putting in a shift kit --- even a mild one, can cause a lot of communication errors and crossed-expectations.
Before I forget --- "Hanging control valves" can be a corrupting factor too, in that if they don't release - or they don't open or apply in a timely manner --- then things go rapidly wrong too.
When you diagnose and repair transmissions --- there are so many things to think of --- all the time --- or you won't fix it right, if at all!