I switched from a Carrera tail to a ducktail and still went fast.
Like Wil said, I think the differences are hard to quantify on our cars at the speeds we typically see on the track. Unless you're at a fast track with high speed sweepers, I doubt you'll truly feel a distinct difference.
Cooling is fine with a ducktail. Think about these couple of aspects
>The cars without Carrera tails (just a plain deck lid) cool just fine
>The opening in the Carrera lid is the same as the plain lid, except the tail has a larger surface area & more slats to capture cooling air.