> How much bloat does ipv6 put in userland ? Did someone measure this ?

I don't know much about the big picture of the build process.

How hard would it be to make a script that would build all or some 
specified subset of userland in both modes and measure the total 
disk space the results used? 

That seems like a handy script to have around.  Aside from measuring 
the space difference it would verify that the compiles actually worked 
in both modes. 

I'd expect a good thrash like that could easily be run occasionally 
when somebody was checking out changes to a disk driver or file system.