> GetValueImpl is a template so it can not be virtual.
Humm … yes, I had forgotten that for a second :(.

> that make me ask you again if we want to go that road
Since the TFormLeafInfo are (somewhat) internal, i suspect we can be backward incompatible, however I do not have a much better idea as (at least for now) the TFormLeafInfo implementation strongly rely on virtual function calls.