"It's not nearly as noticeable as people make out."

This would depend on the tire. On the tires that the OP mentions, maybe you're correct. The lighter and more supple the tire (think racing or sport tires), the more noticable the difference. I'm not dissing Mr. Tuffys, just saying that you don't get something for nothing. If you add weight and a peice of rubber/plastic to a bicycle tire, it's going to feel different. This is true whether it's an add on (like Mr. Tuffy) or molded into the tire (like our Marathon Plus tires).