Pierre Gutelle's "Design of Sailing Yachts" has a really good section on two masted rigs. He quotes a lead for ketches of 11% to 15%.
Although two masted rigs are not theoretically as efficient as sloop rigs, they do break down individual sail sizes which may make handling better, and of course there is some redundancy.
In practice, the mizen needs to be carefully trimed in order to avoid loading the rudder with a lot of weather helm.
With regard to conversion, you obviously need to consider the structural and ergonomic implications to the boat, my experience is that it is quite hard to accomodate the required E mizzen + E main + J inside the deck length of the boat.
Historically racing yachts from the early 20th century were converted to ketch or schooner rig for cruising, however they had longer keels where lead was less critical.