Userspace services daemons etc, like any program will
perform better on an
i686 processor if they are i686 optimised :-)
It would certainly be interesting to see comparisons of this. The primary advantage is that more registers are available so there are less games played with swapping values in memory for those routines which contain many variables. Personally I have written very few routines which exhaust even the small(ish) number of registers on the i386. If you believe you have a significant gain in compiling for the i686, you can always use the Debian tools to build the same packages from source.