I think upping the tire sizes is a response to R.E. choosing tire size by price point for the masses.
The stock sizes aren't the minnimum that the rims would fit, but they are on the lower end. The sizes I'm going for are above mid-range for the rim widths, but they're not pushing the limits by any means. R.E. chose 2.5 front and 3.5 rear so we could upgrade if we want to. They chose aluminum rims with stainless spokes and nipples so the wheels would have a long lifespan in a variety of climates.
Honestly I think it'd be interesting to pick up a high mileage Interceptor in the future, ditch the 18s for 17s, ditch the FI for carbs, ditch the ABS, ditch the ECU & go with a custom wiring harness, small light battery, LEDs everywhere and all of the other lightweight bits. That could be fun.