I just upgraded to a larger mash tun (moving from a 5-gallon round cooler to a 9-gallon Coleman) and for my maiden voyage I wish I had held back a third or more of my strike water before adding the grain. To my surprise, the temperature didn't drop as much as I am used to (I am guessing because there was more hot water... this was also my first no-sparge batch... so it was a large amount of water, at least for my typical brews... cue foreshadowing music...).
Edit: the other change was that to fill my orange cooler, I would pour in a gallon of hot water at a time. To fill my new cooler, I drained directly from the liquor tank via heat-resistant hose... no doubt also contributing to less heat loss.
Obviously I should have said to myself, "Self, you are introducing a new variable [edit: TWO new variables...] -- use a little judgment and think through what might happen," but instead I found myself trying to quickly push the temp from 160+ to 154 in my now-almost-brimming MLT at the same time that the guys from Home Depot suddenly showed up to take away the old dishwasher that had been sitting in the kitchen all week and I was locking up cats, lowering the temp in my MLT, etc. (The Home Depot guys never asked why I was pouring water into a cooler filled with "stuff.")
I ended up with too much wort, which instead of holding back I drained into my kettle... forcing me to bail some out and then tinker with the too-low gravity... it was one of those brewdays. When I give talks about organizational leadership I use homebrewing as an example of "give yourself a safe place to fail." Homebrewing has been that for me and then some. Of course, now that I have a blue cooler, that will be a buffer against some of my brewing escapades.