in the manual its not supposed to be the case
they say: 2nd slot share bandwidth with slot 1, so if u used slot 2, the slot 1 will be at 16
but i have nothing in slot 2
the revodrive is in slot 3
hope its clear enough

so yep, something is wrong with :
- GPU-Z
or
- the mobo (puting my slot 1 in 8 instead of 16
or
- the card messing with the bandwidth
or
- the revodrive messing with the mobo and/or bandwidth of the gpu
?