MLT Temperature Question

I am going to be brewing my first AG batch soon and have finally gotten all my equipment together. I have been doing "test runs" with my new equipment to get more familiar with how to expect them to work/operate (more or less). I discovered that with my new IWC that has a pump for using ice water (I have a deep freezer so ice is not an issue) can get 5 gallons of boiling water down to 70 degrees in approx. 20 minutes so hopefully chilling wort will be similar.

I just tried doing a test for my MLT to see how it holds temperature (single infusion single sparge)and seems like I'm losing heat somewhere. Its a 5 gallon Igloo cooler with a SS braid and SS ball valve setup. I added 3.5 gallons of 175 degree water to the MLT and covered it for 10 minutes to completely heat the inside of the MLT and then stirred until the water temp came down to 156 degrees. Once the water was at 156 degrees I put the cover on and let it sit for 60 minutes. When I checked the temp after 60 minutes I found that the water had dropped 5 degrees. I did two separate tests like this and both were the same with a 5 degree loss over 60 minutes.

1) Is it possible that a Mash (water & grains) will hold temperature better than just plain water by itself? In other words will the 5 degree loss most likely not happen when mashing grains vs. me doing a test with just plain water?

2) If I still get this loss of 5 degrees with my mash how much will the brew be affected? It will be a Brown Ale and I like the medium to full bodied type so I imagine I will lean towards the higher end of the temperature zone.

Thanks for the help!

The grains will hold temperature better than just the water. My mashes hold temp to within 1 degree over 60 minutes easily. I wouldn't worry about losing that much heat during the mash, but your first time will be the test. I would assume you're going to hold the temp, so put it where you want it (148-152 will give you more fermentable sugars, higher in the 150's will give you more body and malt), and then if you lose some you'll still be getting sugars vs. if you don't and you started high, you may be shorting yourself or over-malting your balance.