Well it ought to be easy enough to test... Up a VIII, set fuel burn extremely high as you fly around, drain the wing tanks, then coast while you set fuel burn to zero then test with full fuselage tanks only. Then with burn still at zero test the XVI at full fuel (no wing tanks to burn).
In this game, at least, the 16 has a big benefit for acceleration. It's a frikkin' UFO (with laserbeams on its frikkin' head!), and it seems to pull moves that the 8 cannot.
However, going back to the thing I mentioned before... If they are the same, why do we have both? Seems redundant (again).