Also keep in mind that your car needs to run a bit hotter than 180, or it will not come out of cold start strategy and your fuel mileage will drop big time. Most folks running a 180 without fan mods found their cars ran hotter.
The low speed fan comes on at 220 degrees and shuts itself off at 217. The high speed fan comes on at 227 and shuts off at 225 with the low speed fan coming back on at 224. Now think of things this way. Our rads are too small to effectively cool continuously flowing coolant without some assistance, or you need to create a "linger" time in the rad for some of the heat to be drawn out of the coolant. So your travelling down the highway at 55 mph which means the low speed fan will not come on as it is speed restricted at 47 mph, so the only method of getting air through that rad is what is being rammed up from the air dam at front. So the 180 has opened up and now you have continuous flow the entire time you are driving as that 180 will never close once it has been opened up. No fan, no lingering time in the rad and not alot of air flow = temps are rising, and rising and rising with no assist from anything until you hit 225 degrees. (High speed fan is restricted at 97 mph)
So lets take the same scenario with a stock 197 tstat. So your driving along and at some point the 197 opens up and cooled coolant is allowed into the motor and because of this the tstat will start closing again, slowing the coolant flow in the rad which creates a lingering time in the rad to transfer heat out (you hope). Ironically, with a normally driven SC that is stock, this seems to be about the optimal temperature for the entire cooling system to work efficiently and be stabilized , so the 197 keeps opening and closing allowing that lingering time.
The recommended fan on temps for a 180 tstat is not at 180, but just above 200 degrees. Dave Dalke figured this is the optimal running temp of our motors. You will also note, that most folks that are going for more power, will generally run a griffin rad which of course offers alot more surface area to allow heat transfer.
I run my car with a stock tstat, 50/50 mix of coolant, and using the EEC tuner I've dropped the low speed fan to 200 and the high speed fan to 207. Even when I am driving very hard etc, on the days we see 110F (We've had that 3 times here this year so far) with the humidex my car does not see over 200 degrees. I actually have to test the high speed occasionally to make sure it still works. I also keep my entire cooling system in pristine condition. I would not even hesitate taking my car the way it is and go to Phoenix and drive her hard on the days with 120F temps. BTW all these temps have been verified with an after market temp gauge as I would NEVER trust the one in the dash.
If your going for high performance, then use a 180 with a better than stock rad and a tuner chip to lower the fan on temps. IF your car is staying stock, the 197 is fine so long as you keep your cooling system in pristine condition with a proper mix of coolant and water.
Anyways, gotta go an enjoy another hot day, as estimates with the humidex are around 112F now