On my friends car all it was was a .10 difference. This is the same day, same track, same size tire, about an hour or so between passes (he borrowed another buddies tires for a pass to try them out). My friends car has a killer suspension setup just for drag racing and the whole car is setup for drag raceing (no backseat, no sway bars, no PS, no AC, cage, battery/fuel cell in trunk, etc). So it didn't make a whole lot of difference on his car. He was hooking good either way.
Now on my car (significantly slower then my friends and I'm on a stock suspension with Eibach Sportline springs) MT's would make a world of difference. I may pick up .4 or .5 over the 555R's, maybe even more.