My mechanic put in a hotter spark plug for my car since i run premium fuel tune via diablosport.....Its 2 ranges hotter.....if i switch back to 87 octane tune is this bad? What are pros and cons of a hotter plug

 

Just listen for detonation, as long as you don't have any you are okay. Hotter plugs hold more heat and therefore it makes it easier for the ignition to fire.

 

with a 2 heat range hotter plug- if your car is tuned for premium running that low of an octane gas will cause detonation(pinging). I personally wouldn't run anything below 91-93 octane with hotter plugs.

 

You want to use the coldest plug that will run without fouling.The only way to figure this out is to drive the car for a few days and pull out a few and see if they are starting to foul or are running clean.A cooler plug will allow you to run more advance without detonation.
If you are running too cold they won't last a few days and the engine will start to get a miss.