Depends what qualities they where bred for Rocky. Most 2nd/3rd etc i have seen have been bred for big bucket heads and aggression, not so much to do with speed and ability. Not all granted, but a lot.
Gafer, your 1st gen, 1st cross are a different dog imo than line bred 1st x's, just something for people to be aware of when they say they want a 1st x.
i dont have one yet buddy butt yes i no a bout 2nd 3rd and 4th gen breeding i have a 4th gen 1/2x dh x gh at the mo and wont a bitch so later in life i cn put him to it