Is your lab used to exercising in the heat of the day? It all comes down to conditioning and mitigation like everyone else has stated. Can't take any house dog out on a dove hunt that has been sucking down A/C for the last few months and expect them to do well.
I have a lab that has lived in Texas and now Bangkok, TH...... and I thought Houston was hot....
Exactly. My 9 year old yellow lab has lived, breathed and worked outside during all kinds of heat. She has fans, and at times misters, and often a tub to get in and out. She's a retrieving machine, and at 9 may be slowing a little but not much. Wednesday evening out west was hot and humid for that world, she drinks as much as I'll give her. Get them down and calm in between. Don't shoot flocks, in heat I just shoot singles.
But training and practicing and living in heat makes a difference, same as for human athletes that I coached for 30 years. Proper training, conditioning, and preparation prevents more problems than you can ever solve.
I chose yellows because I hunt dove more than anything else. They are dove dogs. I can see where black/brown labs would have more heat issues.
As far as the cool mats, etc... if you have to do that, cool from the top, cold sinks, heat rises