GM info indicates 100 RPM reduction of engine speed when the TCC is engaged. Road test with TECH2 in hand shows as much as 175 RPM's. Some line of codes in the VCM/PCM in certain years and models show the ability to achieve lockup in 2nd, 3rd much less OD.
For fuel mileage loss, all kinds of things can eat away it. Write down mileage and gallons to the tenths and repeat to make sure.
Without any check engine light, try using Techron by Chevron. The trick is this is safe in the gas tank, not true with GM / ACDelco X66P Upper Engine & Fuel injection Cleaner."
Shop around to find the Techron, then with a low tank, empty 1 bottle to increase the ratio fo cleaner to fuel. Then drive and it will clean injectors that improve the spray pattern. It can't hurt.
It used by fuel supply companies as the only approved fuel injection cleaner (5 or so Car Manufactures signed on) added prior to tanker delivery. It is measure at an exact ration to qualify as "Top Tier" fuel.
Now, online shopping may bring up the GM/ACDelco X66P. Be warned, it is for injector rail only and not to be poured into the tank. Fuel pump damage will occur. I have the actual kit that allows for a pressurized tank, that fuel with 20% X66P is added to fuel. With the return line is plugged, the engine is run until it stalls. EGR plates are provided for direct cleaning. X66P will remove the carbon from the intake valves, but if applied thru the EGR, it is dispersed to all cylinders equally therefore it will clean intake runners as well. GM's TSB / Warranty indicates when a customer has a properly diagnosed restricted injector, it is to be cleaned and varied as repaired, the pay the claim. But, the owner is instructed to use "Top Tier" fuel from now on, otherwise there will be a charge.
So try a few normal things, cleaning the air filter, MAF is tricky but can be done, throttle valve, IAC passage, tire pressure, tire wear, etc.
I will never forget the days decades ago at the dealer, when customers asked about why the transmission felt like it was shifting "TOO" much. The explanation of TCC satisfied some...others not so much. Then came the crowd of men who where convinced they had a short, as certain electrical things stayed on when the key was off, open door, went off. "Sir, its retained accessory power." Then came "Gas Cap" CEL. Got the service advisors to tighten the cap or check it before writing a repair order that dos not pay.
Well, hope this helps. And perhaps a little laugh.