id still get the ECU adjusted for the tire size... even though you are fine with having GPS speedo, its not accurate and updated fast enough and your mileage is still going to be off (meaning you will have higher miles registered than you actually do). The pro-cals you can usually find cheap online or on craigslist so no reason to never get one, especially if you plan on doing something later.
Also, the mechanic may not be an idiot, but he may not have explained enough or correctly. With an auto transmission, the shift points will need to be adjusted with larger tires. Not really *need* but should. Procal's help with that too (so really is a good investment).