It seems like it's hard to definitively say one way or another. I experienced significant slipping mash temps early on in AG brewing that seems to be the cause of over-attenuation. My experience would indicate that significant slipping mash temps do cause overly fermentable wort BUT the circumstances are probably equally as important. Time to denature, amount of temp drop, how quickly temps drop, duration of mash, etc all play a factor, and is why I think it's hard to have a definitive answer to the question.
BTW, I use a cooler and my temps slipped significantly in the early days of AG brewing so even with a cooler it can happen.
I've resolved nearly all temp slippage by simply using a temp refreshing step at the midpoint during mashing. I simply reserve 0.5-1.5 quarts of my strike water in a pan. At the midpoint in mashing (usually about 30minutes) I heat it to boiling and stir it into the mash. This gives me the opportunity to 1) stir up my mash, 2) check how much temp drop I've had, and 3) refresh the mash temp to where I want it. Additionally, I assume that whatever slippage has happened has only lasted a short while.
At least, that's been my experience.
I've never experienced a big temp drop in coolers - I've used used the Coleman Extreme type coolers for years. How much of a drop do you see, out of curiosity ?
I mash indoors with my batch sparging cooler setup. Using a 5 gallon rubbermaid water cooler I was seeing probably about 6°F drop over the course of an hour, but I believe several things contributed to this. As an example, I'd start at 154 and end up at 148 (which puts the midpoint about 151F), and most likely I went from 154 to 151 fairly quickly (within 10 minutes) and no where near as quickly from 151 to 148 (remaining 50 minutes).
I wasn't accounting for the immediate loss of heat from the cooler equalizing with the mash (thus the reasoning for the quick initial loss). Obviously, I wasn't preheating the cooler in any fashion. I wasn't aware that my (probably most) lids don't seal very well. Lid was not insulated. And, I prefer to take an average measurement of my MLT temperature as opposed to spot temps (explained below). Long story short, lots of mash temp loss and over-attenuation of many beers. Since realizing the limitations of my system and resolving those items that I could easily resolve, I have not experienced such over-attenuation that I once did (with the exception of using new yeast strains that over-perform what the manufacturers state - 1187!).
As for my temperature taking, I would see what I would consider significant difference in a temperature reading from the center of the mash versus a reading from the mash near the cooler wall (a couple degrees). Enough so that I decided the best way to get a reliable and reproducible reading was to stir up the entire mash (walls with center, top with bottom) while watching the thermometer (rt600c). This approach works for me and I feel that the mixing of the "microclimates" inside the MLT gives me an accurate picture of what kind of environment is being produced so I have a better idea of what kind of enzymatic activity could be taking place.
Since refining my process, which can certainly seem overly complex/critical/dorky, I am seeing attenuation in the ballpark of what I'd expect - most times. Sometimes I'm thrown a curveball that I try to figure out (dark, roasty beers), or a new-to-me yeast strain that I need to become familiar with, but it's all part of the fun of homebrewing for me.