Hm.. let's do some math. The offset is around 10cm to fit next to the LNB and the lever arm (focal length) of a f/D=0.7 is 80*0.7=56cm. So the angular offset is arctan(10/56)= 10 deg.
According to G3RUH (60cm dish) a 60cm dish has a -3dB beamwidth of 16 deg, so 8deg offset is -3dB. The 80cm dish should be more focused and we are 10 deg off, so I would at least expect -5dB or ca. 1/3.
So your quoted 8-10W fits nicely with the 2-3W I was hoping for with a perfect setup.