I agree with this and will have mine shut off soon (BBP on the way!). Now that my exhaust is louder, I can hear when it shifts from 4 to 8 and that's the main time I notice any weirdness. I also turned off grade breaking for the first time today and it does make a difference on the low speed issue in the original post (on mine anyways).
My thinking on warranty is that both of my transmissions that have failed on me happened a little over 100k, one GM paid for half a rebuild and the other (125k miles) was all me. Both had 100k powertrain warranties. So most likely even if you have strange shifting or the "C clunk" as my service department called it on my 09 the transmission will, more often than not, last through the warranty. Even if it fails under warranty they have to prove something you did caused it, flash it to stock and take it in see if they fight it. My understanding is a well done tune can actually help the longevity of your powertrain, as long as you don't run it like a race car anyways.
Back on point, mine is a 5.3 8spd 2016, and I do like the transmission a lot more than my 09 6spd. The weird shifts are not real common for me and none have been as severe as the OP described.
Sent From My Galaxy S6 Edge+