Changed the thermostat and still running under 90°

This was the post that made me think you were getting SDLs, sorry for the confusion ...

Yes, I used to think the coolant temp should stay steady like all other cars too, but we now know that is not the case on the 355. So if you are within 8-11 for most driving, then you are OK (+ / - one click) ...

 

I have fabspeed headers and fabspeed test pipes. So there really isn't going to be hot headers with no cats, which is a good thing. At this point the extra wear from cylinders washing out is my biggest concern. After that there is something to be said about a complete burn creating more power and better fuel economy. That would be my secondary concern.

I also don't understand why such a cooling system would drop from the ideal range so far. Is it just the thermostat in control or does the massive amount of coolant and double radiator system of the 355 just cool the coolant down to much when it passes through? I have never owned a car where the needle moved as all after up to full temp.

 

Yes, those are valid concerns and a rich mixture will indeed hasten cylinder and cat wear, if left uncorrected for extended periods.

Ok with unshielded fabspeed headers it is much easier to check temps. Measuring exhaust temps is an additional method of determining a coolant temp problem. With a hand held digital pyrometer, measure the point 4-6 inches after the 4-1 collector. Where the manifold goes to a 'single tube' (just before the axle). Soon after a full warm up, the temps should be between 400°-500°F and just as important each bank should be very close in temp to each other, not more than 50°F delta between them. The closer in temp to each other the better!