probably too late for GerardG, but just did Rumphi (Malawi) to Uvinza (Tanzania) with a riding buddy, mid-November 2015.
Rumphi > Chitipa (Malawi)... dirt, thru Nyika Park in Malawi
Chitipa > Tunduma (Tanzania)... dirt except 25 km or so at the end
Tunduma > Sumbawanga ... all tar
Sumbawanga > Uvinza ... virtually all dirt
At Uvinza we turned east and made Dar es Salaam a few days later.... a mix of dirt and tar from Uvinza to Manyoni, then tar all the way.
All dirt roads were fine & doable except for a bit of mud as the rainy season started up for the year... no sweat in dry conditions, at the peak of the rains I imagine those roads would be a bit trying.
Remote western Tanzania is an awesome adv moto destination, be sure to camp or lodge at Riverside Camp in Sitalike, Tanzania, at the north of Katavi Park, next to 300 hippos!