that's actually a myth that TT guys tend to tell n/a guys. Unless you have tested an n/a car both ways, we wouldn't really know. That seems to be a common myth though, I suppose it might depend on the rest of your mods, but I've never seen a car slow down in 0-60 due to losing it's cats, turbo or non turbo, it may feel slitghly sluggish until your car's ecu adapts to it after a few runs.