I'm wondering the same thing. I bought a racedeck floor during the Black Friday sale and have them sitting in boxes on the garage floor awaiting installation. Of course, I want to paint the walls first, which means finding a place for a stack of hardwood piled alongside one wall and getting warmer weather in place, so it will be a few months before I get to it.
Still, I would like to know whether it's worth it to buy and lay that foam underlayment. I put down several hundred feet of laminate flooring in my basement a few years ago, so I am on familiar terms with it, and it's not all that expensive if it makes a difference.
So, I second the appeal to anyone having real-life experience with garage tiles with a foam underlayment.