I actually tested all the Gr race cars at Tokyo East's main straight a while back. The Atenza is defo the slowest Gr.3 (266 kmh). Above it are RS01 (268 kmh) then Z4/Evo/WRX/RCZ (270 kmh). The top speed kings are F1 GTR (285 kmh) and Pug VGT (283 kmh). But obviously those have poor handling & brakes. The "handling" top speed cars are as you suspect, Supra/FT-1/GT-R/Jag/Ford GT, which are all pushing 280 kmh. SLS and old M3 are quite fast too but obviously their handling is not as good as the newer cars. All other cars are somewhere in between. The most balanced car is probably the Beetle, mid level top speed (275 kmh), good easy handling and one of the best tyre wear (only chink in its armour is poor fuel consumption, but then it has 7 gears so it can short shift easily).
The caveat with the Atenza though, is that it has the most powerful BOP'd power. So acceleration out of corners is actually very good and if the straight is <400m you can still hang with the big boys, which masks the low top speed at technical tracks. And of course, with how OP the draft is with Gr.3, if you can stay in draft that will also mask the top speed deficiency. I spent like 2/3 of the race in the draft, and got a perfect tow down Dottinger in my quali lap, lucky me
The Gr.4 Atenza, I agree is slow as molasses, but in reverse to the Gr.3. Top speed 264 kmh (Evo/WRX is 260 kmh), but acceleration is very slow. There are slower cars (Megane Trophy, 4C, 86) but those are OP handling and tyre wear unlike the Atenza which has no redeeming features. None of that applies if you're an alien though, as Rangeraus posted a few pages back akagi-san posted a top 10 time with the Atenza at Bathurst