80D vs 90D - threat protection throughput

No, that's correct. The FG-80D is especially built for high UTM throughput. Specifically, it's CPU-based whereas the FG-90D is a SoC-based model. Fortinet's datasheet figures are quite sustainable - a combined value for a mix of UTM features is quite difficult as the traffic composition heavily influences the outcome. Still, for comparisons these figures hold.

If you are considering to purchase a FGT choose one of the new E series models. For instance, the 60E is at least at par with the 90D though it doesn't have an internal SSD; the FG-61E does. You will get better performance for the buck here.

I personally never really liked the FG-90D. At a pinch, I chose the FG-70D which is a diskless FG-90D. YMMV.