It shouldn't matter that the torque converter wasn't filled. When you crank up the engine, it should fill right up. I just had my C6 rebuilt and the guy told me this........42 years in the business, I just followed his directions.

What is your stall rpm on the converter? When you filled it, did you let the engine run, put the gear selector in all positions and let it sit in each position for a few seconds to let the fluid in to all of the inner workings? What is your idle on the engine set at? It may need to be raised depending on which converter you have. How much fluid did you add? It's not uncommon for some systems to take quite a bit. My C6 has just over 3 gallons in it. Also, the quicker the gears engage, the more you know the fluid level has pretty much filled or gone through your system after it has been completely dry. When first filling the system, it might take a few seconds before they engage because it was dry, initially.

Also, just for the heck of it, if I were you I wouldn't hesitate to call the shop and ask them their advice...you did pay them to set it up correctly.