I've found that it really depends on the 'type' of dirt road your talking about.
I've been on some that are smoother and faster that the shyte tarmac roads we have in NSW and managed to do 100km/hr, but other rougher tracks its down to 40km/hr.
The only thing that really 'scares' me on dirt with the B12 is the power, you've got to be really really really gentle with the throttle..
If you're contemplating doing a lot of dirt roads, I'd be getting an adventure tourer, something like a BMW GS, Aprilia Capanord, VStrom or maybe one of the smaller 650 singles like a BMW F650, Pegaso, DR650 etc.