1) personally I think this is a rare case, but I could be wrong
2) arc2c closures are very lightweight: it's just a simple array of obj(s), with the first obj being the pc. So in effect for functions having no environment, we are pushing a pointer to the pc.
That said, closures are also used to represent functions that can be passed around. Unfortunately closures are currently untyped, so we expect the current closure style to be changed.
Also we need to support the possibility that a "function" being called isn't really a function: after all table syntax is just (tb key). And this is perfectly valid Arc:
(let sometable (table)
(each k lst
(= sometable.k (generate-something k)))
(map sometable ; yes, we're passing a table as if it were a function!
3) I was actually thinking of this too, although I haven't gotten around to it.
re: CPS: I wouldn't really know. Me, I'm just hacking around at the transformations before the CPS and Closure conversions. Because of the somewhat modular construction of arc2c, in theory you could write a drop-in replacement for CPS and Closure conversions, as well as code generator, and we can then put either CPS or the direct style as options, maybe.