It reads like you have only 7 days for the whole expedition
If the week of riding includes the run down from Toronto you will not have very much time in Mexico . I know it takes me 4 days of riding from S Ontario to the Tex-Mex border and that is usually the longest riding days with little attention to scenic routes. From TO it will be the same story- 401 to Windsor and then interstates to the SW . If you want scenic routes in the US I would take freeways to mid -Missouri and then pick one of the squiggly black lines of state highways south thru the Ozark plateau into Arkansas and west through the Ouachita Mts, then into OK and back on fast freeways south thru Dallas-FW I 35 all the way to the border.
That leaves about 3 days for a quick loop in Mexico before you have to fly back.June will be really nice for the ride,longest daylight available, no worries about cold weather or heavy clothing, the countryside will be all in green and cactus should still be in flower.
Since you are talking of flying , think of the cost of shipping bikes and your flight ticket . Compare that to the income you would loose if you were to take another week extra- or two weeks, if that is a possibility. - Does it make sense to rush like your plan would require or would it be possible to stretch your free time.? One week is really not enough .Maybe you can have mor e free time available at another date so you could ride down and back?If that is not possible and finances are not a limiting factor how about getting the bikes shipped to Laredo TX before the trip and just fly down an back with a bike trip given the full week in Mexico..?

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93