I'm thinking the 300M delivers lower mileage because of a lower (higher numerical) gear ratio to boost performance, and/or wider and stickier performance tires with higher rolling resistance than the more sedate models.
If you want to maximize mpg, I'd try at least an extra 5 lbs over factory spec in the stock tires, then replace 'em with no higher than H-rated rubber when the time comes. Also suggest going to a lighter grade of synthetic oil, and putting in a K & N air filter, perhaps with a cold air intake kit if one is available. Also try to find the most efficient cat-back performance exhaust, now or when the tail pipe and muffler go.