Michael Yardley is one of the UK's top gunwriters, shooters and general defenders of the faith. Below is a link to an article giving his perspective.

One of the issues he raises, that gets overlooked too often, is that beyond the question of how steel affects the gun, how would it affect the sport of clay shooting?

As he writes, "We might also consider the implications to clay shooting. If lead were banned, sporting clay shooting would become a very different game. The challenging, rangy, birds seen at today’s shoots would have to go – you just can’t hit them consistently with steel. Some forms of trap shooting would be severely affected too."

Yardley's article: http://www.positiveshooting.com/NonToxicShot.html