The difference (in work) between map Wrapped and conv is the difference between map id and id :: [a] -> [a]. In the absence of any fusion/rewrite rules, the former breaks down a list, and builds up a new one with exactly the same elements (or, every element x becomes an id x thunk, perhaps). So, in a lazy language, inspecting each cons cell carries an additional O(1) overhead over inspecting the corresponding cons cell in the original list (because inspecting the former implicitly inspects the latter, and then yields a new cons cell with the same values for inspection).
On a related note, I've been occasionally worried about conversions like 'map convert huge' where 'convert' is from one newtype to another with the same underlying type. I tried some simple examples and looking at core it seems like the 'map id huge' is optimized away. However, I'm guessing that's only because of a 'map id xs -> id xs' rewrite rule involved, and it won't work for all data structures. It seems like a better solution than relying on rewrite rules would be to lift the newtype up one level, e.g. convert 'M (Newtype x)' to 'Newtype (M x)'. Actually what I really want is to replace every function that goes M x -> x with M x -> Newtype x, but we don't have parameterized modules and doing this for something like Data.Map means a lot of boilerplate. Surely there is some more general approach?