You might check dealers work order to see if they used correct fluid stated on page 17 on this TSB.

MC-10163890-9999.pdf (nhtsa.gov)

 

You are in the wrong section with a 2015.

But the torque converter clutch can be weared out. Sounds like your converter lock slips which leads into rise of rpm, if l understand you right. As well your transmission temperature rise, because of the permanently friction and wasted energy.

 

Our 2015i with the 8L90 had a torque converter failure right at the end of the warranty period, thankfully. Dealer tech found that it wasn't locking up properly, so slipping and possible creating more heat, 100K on the odometer. Dealer replaced, no hassle. But you are right, those temps are too high. Normal range for our truck is 175-185, nothing being towed.

 

Here is article of what they call ''pill flip'' to lower temp in trans,

6l80e Thermostat Delete - How To Delete 6l80 Thermostat (transmissioncoolerguide.com)

 

i recently got a kit and installed it from summit racing for around $12 that allows the fluid to bypass the thermostat and it was for the 6speed or 8 speed

 

Hey! Thanks! I went to the dealership and purchased the entire transmission thermostat housing. I quickly learned that GM had modified the part which would require me to replace the coolant lines connected to the trans. With the housing and new lines I was looking at $300 bucks not counting labor had I let the dealership do it. I went on Amazon and purchased the Supperior Thermostat Bypass Kit and it worked! The temps are normal! I still have the shuddering, but I plan to address this in the coming week. Thanks to everyone for your feedback.