From others have experienced, no - you won't gain anything (substantial) on a stock cobra by swapping in a lower thermostat (due to the computer/PCM). You're probably better off wiring in a manual cooling fan switch or getting a chip if you want to tweak with the temperature at which the fan is set to turn on at...
For those from the old school one would think swapping out for a lower temp thermostat would be a good thing, however I have found that with the 4.6 DOHC engine and many other modern engines there is no benefit as they are designed to run warm for efficiency and emmissions purposes. In addition allot of the internals such as pistons and rings will wear prematurely if not run in the optimal temperature they are designed for. Since the EEC controls when the fan turns on and off to match the stock thermostat of 192-195F you would need to chip your car to reprogram the fans to really do any good with a lower thermostat. There is allot of debate about your subject but that's my .02 regarding a NA or stock engine. If you build out an engine, add a power adder or juice than it could be another story.
Also, the PCM doesn't lean out the mixture from warm-up mode below 175 degrees or so, so it'll always run rich....unless you can chip to allow for it.
I think the thermal expansion of the block, etc. is probably "close enough to go" around 160, so if you can chip to run the optimum fuel mixture at a lower temp, you will save a minimal amount of wear on the internal components.
You can go with aftermarket gauges (either electric or mechanical) as well. While NA, oil pressure & water temp gauges are probably the most common/helpful gauges to install.