Mishimoto offers a 180 degree thermostat for the C7 for $50. No need for a new housing. The thermostat is removable. It you don't drain the radiator you will wind up with a nasty mess on your hands.
.
https://www.mishimoto.com/chevrolet-...ml#prettyPhoto

 

Thanks for the suggestion. I have a stock thermostat and housing ($68 + shipping) enroute. Will be delivered in a couple of days. I have the tracking number. I prefer stock since mine is still under warranty.

 

I live in Treasure Coast Florida and had similar issues with my 17 Z51 M7. If I started it cold and went almost directly to the highway the temp would go to apx 185 then you could see the thermostat open and them temp would drop to 170. It would never go above 175 after that and as you stated the yellow would turn back on (on the Tach). I had been wanting to take it in but had not gotten to it then the car gave me a CEL and it had to go in. The CEL was not related to the temp but my dealer checked the cooling system and are replacing the thermostat reporting that it was not closing correctly. With our summer heat this issue might have been around for awhile and just showed up with cooler weather. I typed up a detailed description like you did above the the service writer attached it to the service order. This got the detailed description of the issue back to the Corvette Tech. BTW the GM extended warrantee is covering the thermostat.

 

Here I hope is the last update on this thread from the OP. I purchased the thermostat and housing from one of the many online chevy parts houses (GM Parts Online). Took a couple of days to get to me. I thought it was a little interesting to hear stuff rattling around in the box when I moved it. The outside box was about 6x6x12 inches. I opened it to see another box about 4x4x6 inches inside with no bubble wrap or anything else. When I opened the smaller of the two boxes, the thermostat and housing were inside and the bypass nipple on the housing was broken off. I probably could have used my original housing and just swapped the thermostats around but I decided to call the parts house instead. They were very polite and shipped another thermostat the same day. If I need more parts online, I will not hesitate to use them. I returned the broken one.

Second housing arrived a couple of days later with bubble wrap in the larger of the two boxes. I installed it today. Rather than try and reuse the old coolant, I bought a gallon of dex cool at one of my local dealers and mixed up a 40/60 ratio. Used a friends 4 post lift, drained about a gallon of coolant, removed the bypass hose, removed the thermostat housing, removed the radiator hose. Easy. peasy. Install went just as easy except I installed the housing to the block then installed the two hoses. Burped the cooling system like it says in the owners manual and during that process saw the temp go to 198 then cool to 196. Seems I have a 195 thermostat. Took the car for about a 50 mile drive on surface streets and freeway, temp stayed in the mid 190s. Up to 201 at a stop light. So my initial problem was in fact the thermostat, two local Chevy dealers were unable to correctly diagnose the issue and I ended up changing the thing myself. In the end everything is back to normal.

While I was under the car, I took the time to look at my wheels. No cracks to be found, phewww. But, I am going to need a new front center air dam. the left most tab that bolts to the car is broken or I might try plastic welding just to see if it works.

 

The multicolor ring on periphery of tachometer would keep indicating cold, suggesting reduced rpm threshold. It would go away for a bit, suggesting normal operating temp, no rpm restrictions, but the cold indication would soon return.
I started the car from cold and pointed a pyrometer on the thermostat housing on either side of where I would think the thermostat element would be to determine at what temp the thermostat was opening.
When the coolant hose downstream of the thermostat got hot, i pointed the pyrometer at the thermostat. It was 125 F degrees. Opening too early. That's why car wasn't reaching full operating temp. Replaced thermostat assembly and the problem is gone. Thermostat assembly replacement very easy 10 - 15 minutes.

 

I have had all the previously mentioned symptoms. On cool days with grocery store driving the caution band on the tach would only go out
if I was sitting at a light or driving it harder. On the track was no issue.
I feel that 160 is too cold and didn't care for the fact that there were few options for temps. So I went with a BTR aluminum T-stat housing that accepts an LS T-stat.
I'm happy now.