The updated patch i submitted included the following changes:
    1. More explanation of listcomps in the docs.
    2. Modified test cases to use [(x, y) for ...].
    3. Added a test case to ensure [x, y for ...] is a SyntaxError.
    4. Cleaned up grammar by adding a "listmaker" rule.
    5. Temporary local name is now "__1__" instead of "1".
    6. Look up "append" once, outside the loop, instead of inside.

The optimization in #6, which is one of the things you were worried about, produces a noticeable speedup (up to 20% on very long lists).

Skip, i updated my tree & it looks like not all the patch got checked in.  Did you check in Doc/tut/tut.tex also?