I just think that your complaint is strange to me. That after over 20 years of working on them professionally. And driving a few thousand cars. Lets say around 2500-5000. I really could not say in all those years. And I have owned at least a dozen cars over the years with the transmission. That not once have I found that they were inconsistent or that anyone ever complained about them being inconsistent. I have had a few sporadic complaints that were a direct result of a bad part that was fairly identifiable. By the very nature of the hydraulics they cannot be inconsistent if they are in good shape. A bad seal or worn bore or a something that is leaking more when cold vs hot or vice versa could give you different results but again this should be fairly identifiable. And again not something that any customer or car that I have ever had or driven has had a problem like this. Not to say that it could not happen as they are getting older by the day and what was never a problem 10 years ago can be a big problem today.
And I am sorry but no GM transmission last for 30 years without being rebuilt a few dozen times.