I suspect the mechanical gage is at a different pick-up location than your add on gage. I doubt the larger wire will make more than at most a few deg change.
The temperature probe is really a variable resistor so adding a fixed resister in parallel while sounding good and may get you the correct reading at one temperature all other readings will be off based on the fixed resister.
If you know the new meter is off then put a mark on your new gage (red dot from finger nail polish) where the normal operating temp is based on the mechanical gage pretty easy to tell if it reads above the red dot you are running hotter than normal.
