The Mercedes Benz Sportline suspension dropped the height of my car about 3/4" to 1".
The conditions you describe are pretty extreme. 15 cases of soda comes to about 300 pounds hanging out behind your rear wheels - I am not surprised that your car was rubbing on the tires, especially with 18" rims. You didn't mention rim width or wheel offset, which will also have effects on rubbing. Those are some fat tires, if they are on 8" rims you may need your fenders rolled. They are nearly 1" larger in diameter than your stock size tires, and already have cut down the fender clearance by 1/2", besides introducing ~4% speedometer error.
I personally would not install the Sportline kit with only the stock springs, as it was designed as a package, and I don't know what the results would be with stock springs.
The Sportline parts are somewhat stiffer. MB designed a package that would preserve ride comfort and vehicle utility
while improving handling. My opinion is the existing wheel and tire size are going to have much more effect on rubbing while loaded than will the new suspension bits.
I would go for the Sportline, verify the proper size tires for your application ( http://www.miata.net/garage/tirecalc.html
), roll the fenders as needed, and avoid carrying things like short block Chevy engines in the trunk!
[Edited by JCE on 07-06-2001 at 12:34 AM]