I have experience with both: multiple deeply-integrated cons objects that satisfy the consp function, as well as allowing non-cons objects (including classes in the OOP sense) to take operations like car and cdr.

Once you merely go from one cons type to two, with their own tags, every place in the run-time which checks for a cons cell has to now check for two possible type tags. An atom is everything that is not a cons, as you know, so that function is also affected.

(I wonder whether it wouldn't be better to just have one cons tag, and use some flag field to distinguish lazy conses.)