I was on GT675s with and without breathers. They do make a big difference, but the car will still feel laggier than your sport500 setup. you will just have to learn to adjust your driving style to stay a bit higher in the rpms when doing any "spirited driving"
But on a side note, I am now on twin GT3076s with 4.10 leader gears and a built bottom end with 9:1 CR and the spoolup/response blows my breather 675RS setup out of the water....not to mention the HP capabilities