Let me see if I can summarize:
- Your car is a 2009 Smart ForTwo
- Your car has thoroughly modern computer controlled engine management
- You don't want to feed it 93 octane anymore, as a way to save money
- You cut threads off one side of the spark plugs
- You think that cutting threads off the spark plugs is why your car runs okay with 87 octane
I would bet dollars to doughnuts that the plug mod is doing doodlety-squat, and in fact what is happening is that your ECU is pulling timing. That is the *real* reason why you are able to run lower octane.
Here is what I would suggest. Go get some new spark plugs, leave them whole, and install them in your engine. Run several tanks of 87 octane in the car and get honest MPG figures. Then run several tanks of 93 octane, again getting honest MPGs. Try to be as consistent as you can regarding driving conditions. Then compare the MPG differences and see if there is a dollar value to running 87 octane.
Even if you decide not to do the comparison, the very least you should do is replace those mangled spark plugs. The sooner you do, the less it will cost you.