Further examination of the source makes it clear that the hashing is done in blocks of 64 bytes.  I was able to hack the SHA256 functions from PolarSSL and got it to do the block hashing just like the current code, with about a 10% increase in speed.  I verified the hashes produced are the same.

I took the plunge and got all the dependencies together and compiled Bitcoin myself to try to get the new hashing in place.  It's odd that you say that a MSVC build decreases the hashing performance, as I've found it increases it.  I'm using Visual Studio 2008 Standard on a 32bit dual core machine, so maybe that has something to do with it.  I went from ~1000khash/sec with the build from the Bitcoin website, to ~1350khash/sec by just compiling the source with Visual Studio 2008, to ~15000khash/sec with Visual Studio 2008 using the PolarSSL hashing functions.

edit - latest binary 2010-07-22
You can get my build here : http://www.filedropper.com/bitcoin-032_3
You'll need the Visual Studio 2008 redistributable to run this, so if it crashes immediately or complains about an incorrect configuration you need to install this.  It includes the modified sources.  I used the latest bdb, which seems to update the database format, so you can't go back to the old client because it can't open the newer database.  I suggest you save your database before you try this build if you want to revert back later.