Open lakes definitely add warmth to the air flowing over them, but when cold air is advecting into this region from the west or southwest, the lakes have little to do with warming here.
Hudson Bay was a bigger deal for a bit, but it's closed up now.
I'd be more impressed with the lake argument if I saw a reasonable study of how, when, and how much modification we see down here. It would not be a simple study...wind speed, humidity, wind direction with height, heat transfers related to lake effect formation (evaporation - endothermic and deposition - exothermic), and probably tons of things that I would have no clue about
I'm sure the effect is non-zero, but how significant is it under different conditions?