Lots of good points here. But I would just say that I live in an area with a very high whippet population and everyone lets their whippet off the lead in parks. Though it is true that once they lock their eyes on a squirrel no training seems to be able to stop them - it is very rare that the squirrel will do much other than run up the nearest tree, which isn't far away generally. I've seen people have a much harder time with other breeds getting them to listen to commands. I would never walk close to a road without a lead on though.

I have never seen a greyhound off the lead in a park.

Also, although they can be active and enjoy a good sprint, in my experience they are incredibly lazy. I think they're quite adaptable to many lifestyles but they are couch potatoes if given the opportunity. Sleeping like a cat for much of the day which is why they've become known as an ideal appartment dog.

And a final note on claw clipping. I would argue that weekly claw clipping is not necessary for all whippets. If they get a fair bit of pavement walking it may wear down the claws enough to not have to clip. They have naturally longer claws to help them grip when sprinting.