Pit bull/greyhound mixes

On this board and a lot of other boards I see people refer to their dogs as pit/greyhound mixes or pit/whippet mixes. I know I've thought that about Boomer - he is very tall and thin but has a big pit head. This seems pretty unlikely to be a common mix.

I also read somewhere that the serious fighting lines of pits are bred tall and lanky to have better reflexes and speed than the short wide dogs.

So is that true, and is it possible that some of our dogs with unknown backgrounds are not mixes at all but have come from fighting lines? Well, I know that's possible - but can you tell by body shape?

Or is it more likely that our dogs are young and have done their growing up but not their growing out?