BTW did you really mean you ran 1.0.46?  I just tried running that,
and it died ignobly.  It had uncertainty about optimizing various
argument types.

These messages are hints for the developers about the optimizations possibilities, not errors. 
Also, installing 1.0.46 ruined 1.0.47.  Any time I try to run 1.0.47
using an explicit path, it complains "fatal error encountered in SBCL
pid 5804: can't load .core for different runtime, sorry".  It seems
they are not designed to be run side by side, i.e. probably a registry
conflict.  Uninstalling old SBCL versions and running a repair on the
1.0.47 installation solved the problem.

Adjust SBCL_HOME env. variable for the actual version you are using. 

So, how fast is SBCL on Windows? I believe that the performance is decent.