Thomthom,
SU uses the correct units when exporting, but there is IMO a bug in the exporter, as the internal scale flags in the resulting DWG file all point to inches all the same, so unless you take care, AutoCad will import the file 25.4 times too large.
I have resolved this by turning all import scaling off in AutoCad. It involves setting a small bunch of system variables, most of which can be accessed through the Units command. I never need AutoCad to scale my imports, so I have set all these to Unitless.
Anssi