I've never seen anything that says the rear end gears have anything to do with the transmission.

Now that I think about it, it would stand to reason that the Mazda would come with higher gears, because it's reputation for strength and reliability, um, stinks. It's not a good transmission for towing.

It's just plain not a strong transmission. Yes, you will find people that put hundreds of thousands of miles on a M5OD, but you will find a LOT more that have had serious problems with it. It's not cheap to fix when it breaks, either.

For anything other than occaisionally towing that much weight, the Mazda will not be good enough. Start saving your pennies now for when it breaks.