On the other hand, it seems to slow down normal list operations somewhat, about 20%

hmmm, since since most Arc lists are now built out of mpairs in this hack, I wonder if it would be any faster to test for mpair's first:

  (xdef car (lambda (x)
               (cond ((mpair? x)   (mcar x))
                     ((pair? x)    (car x))
                     ...