Siats vs Sauroniops

This all really depends more on Sauroniops than Siats. I've been reading the paper on Siats and the scientists studying him seem to have a pretty good idea on the size of the juvenile specimen (30 ft/2.5 metric tons) and projected size for an adult (40 ft/4+ metric tons). Meanwhile, Sauroniops is only known from a single bone, which could come from an animal anywhere between 10 - 13 meters. If the lower number is the case then Sauroniops would definitely be at a disadvantage, while if the maximum is the case then Siats would be at a disadvantage (since carcharodontosaurids are more robust than neovenatorids, so at equal size Sauroniops would out-weigh Siats.)
Not a lower classification; neovenatorids and carcharodontosaurids are sister-families within the group Carcharodontosauria. One is not a higher classification level than the other, they're on the same exact level.

Also, they're not good comparisons for each other's weights. Neovenatorids took the extreme lightness path in their evolution (in fact they're probably some of the most airy and lightest of all theropods for their body size, other than birds), while carcharodontosaurids took an evolutionary pathway of increased robustness. The former fact is actually why Siats looks so ridiculously light at its adult mass.
Not necessarily. Basal carcharodontosaurids actually have smaller heads for their body size compared to more advanced members. (Primitive members also have longer arms and more gracile skeletons.) Sauroniops shows up as a very basal carcharodontosaurid in most studies, so there is a possibility that it has a smaller head for its body size than more derived carcharodontosaurids.

Bulk does not even help much it's already a good size advantage.

Siats win.