It can't run workbench 3.9 simply because it's CPU is a 68000 running at 7Mhz , the same as in an A500/600 etc..
If i am not mistaken it only has 1mb of ram (i could be wrong) .
So i'm guessing anything higher than workbench 3.1 will either not work or else be very slow .
Other than that , the Minimig is a very interesting piece of hardware and i would really love to get my hands on one someday when i have the money .