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Checked in a speed-up for Py2.5.
See Lib/urllib.py 1.169.

The check-in provides fast-quoting for all cases (not just
for the default safe argument).  Even the fast path is
quicker.  With translation for both safe and unsafe
characters, it saves len(s) trips through the eval loop,
computes of non-safe replacements just once, and eliminates
the if-logic.  The new table is collision free and has no
failed lookups, so each lookup requires exactly one probe. 
One my machine, timings improved by a factor of two to three
depending on the length of input and number of escaped
characters.

The check-in also simplifies and speeds-up quote_plus() by
using str.replace() instead of a split

Leaving this SF report open because the OP's idea may
possibly provide further improvement -- the checkin itself
was done because it is a clear win over the existing version.

The OP's patch uses regexps to short-circuit when no changes
are needed.  Unless the regexp is cheap and short-circuits
often, the cost of testing will likely exceed the average
amount saved.

Determining whether the regexp is cheaper than the
checked-in version just requires a few timings.  But,
determining the short-circuit percentage requires collecting
statistics from real programs with real data.  For the idea
to be a winner, regexps have to be much faster than the
map/lookup/join step AND the short-circuit case must occur
frequently.

Am lowering the priority until a better patch is received
along with timings and statistical evidence demonstrating a
significant improvement.  Also, reclassifying as a Feature
Request because the existing code is functioning as
documented and passing tests.