Filled listobject's tp_iter slot with it's own iterator.  
Speeds looping 5 to 10% (YMMV).

Only half of the speed-up comes from using tp_iter.  
The rest came from code tweaking:
-- eliminate the it variable
-- invert the limit test to avoid jumps
-- bypass the GET_ITEM macro to allow index++ to be 
combined with the lookup.