I've uploaded a new version of the patch that is compatible with the py3k SVN branch as of April 14.

The patch still includes the various cleanups I made to the symbol table processing while working out how to make this change (use sets where appropriate, avoid using the same variable name to refer to completely different things, make a couple of syntax error messages more explicit).

The slowdown should be fixed at the cost of a single function call - the shorter the sequence being iterated over, the greater the percentage slowdown that will be. The speed of generator expressions should actually be (very) marginally increased as they now skip the SETUP_LOOP/POP_BLOCK steps in the same fashion as list comprehensions always have.
File Added: new-comps-updated.diff